Index

Biophysics & Imaging — MRCOG Part 1 Deep-Dive Study Document

Target: MRCOG Part 1 Word count: ~18,000+ words Last updated: May 2026

Table of Contents

1. Ultrasound Physics
2. Obstetric Ultrasound
3. Doppler Ultrasound
4. Cardiotocography (CTG)
5. Radiology & Radiation Safety
6. Hysteroscopy & Imaging in Gynaecology
7. Biophysical Monitoring
8. Key Examination Tips & Mnemonics

1. Ultrasound Physics

1.1 Fundamental Principles of Sound Waves

Sound is a mechanical, longitudinal wave that requires a medium for propagation. Ultrasound is defined as sound with a frequency exceeding the upper limit of human hearing (>20 kHz). Diagnostic ultrasound in obstetrics and gynaecology typically operates in the 2–15 MHz range.

Wave Parameters

• Frequency (f): Number of cycles per second, measured in Hertz (Hz). In O&G: transabdominal probes 2–5 MHz, transvaginal probes 5–9 MHz, high-frequency linear probes 7–15 MHz for superficial structures.
• Wavelength (λ): Distance between successive wave peaks. λ = c / f (where c = propagation speed). Wavelength determines axial resolution — shorter λ = better resolution. At 5 MHz in soft tissue (c = 1540 m/s): λ ≈ 0.3 mm.
• Propagation speed (c): Speed of sound in the medium. In soft tissue: 1540 m/s (the standard assumed by ultrasound machines). Higher in bone (~4080 m/s), lower in fat (~1450 m/s), lowest in air (~330 m/s).
• Period (T): Time for one complete cycle. T = 1/f. At 5 MHz: T = 0.2 μs.

The Wave Equation

c = f × λ

Where: - c = propagation speed (m/s) — determined by medium density and stiffness - f = frequency (Hz) - λ = wavelength (m)

Key Relationships

Clinical relevance: In obstetric ultrasound, a 3.5 MHz transabdominal probe provides adequate penetration for a third-trimester gravid uterus but limited resolution. A 7.5 MHz transvaginal probe provides excellent resolution for first-trimester structures but limited depth penetration (best for structures within ~8 cm of the probe).

1.2 Acoustic Impedance

Acoustic impedance (Z) is a measure of the resistance of a medium to the passage of sound waves. It is calculated as:

Z = ρ × c

Where: - Z = acoustic impedance (rayls, kg/m²s) - ρ = density of the medium (kg/m³) - c = speed of sound in the medium (m/s)

Typical Acoustic Impedance Values

Reflection and Transmission

When an ultrasound wave encounters an interface between two tissues of different acoustic impedance, reflection occurs. The amount of reflected energy depends on the impedance mismatch (difference in Z):

• Large mismatch (e.g., soft tissue–bone or soft tissue–air) → strong reflection → bright echo on B-mode
• Small mismatch (e.g., liver–kidney) → weak reflection → faint echo
• No mismatch (e.g., same tissue) → no reflection → anechoic appearance

The reflection coefficient (R) is given by:

R = (Z₂ - Z₁)² / (Z₂ + Z₁)²

Clinical significance: - Acoustic shadowing occurs behind strongly reflective interfaces (bone, calculus, calcification) because almost all energy is reflected. - Acoustic enhancement occurs behind fluid-filled structures (bladder, cysts, amniotic fluid) because there is minimal attenuation. - Gel coupling on the skin eliminates the air gap (massive impedance mismatch) between the transducer and skin.

1.3 Piezoelectric Effect

The piezoelectric effect is the fundamental principle underlying ultrasound transducers.

Definition

Certain crystalline materials (e.g., lead zirconate titanate — PZT, quartz) generate an electrical voltage when mechanically deformed. Conversely, they deform mechanically when an electrical voltage is applied.

Two Modes of Operation

1. Transmit mode: An alternating electrical voltage applied to the piezoelectric crystal causes it to vibrate at its resonant frequency, producing sound waves.
2. Receive mode: Returning echoes cause the crystal to vibrate, generating a small electrical signal that is processed by the ultrasound machine.

Transducer Construction

Modern transducers contain an array of 128–512 piezoelectric elements arranged in various configurations: - Linear arrays: Parallel elements, producing a rectangular image (good for superficial structures) - Curvilinear (convex) arrays: Elements arranged on a curved surface, producing a sector-shaped image (transabdominal O&G use) - Phased arrays: Small footprint, electronically steered beam (cardiac, transvaginal)

Key Components of a Transducer

• Piezoelectric elements: The active crystals
• Matching layer: Reduces impedance mismatch between crystal and skin (improves energy transfer)
• Backing (damping) layer: Absorbs backward-propagating sound, shortens pulse duration (improves axial resolution)
• Acoustic lens: Focuses the beam
• Casing and cable: Electrical connections and protection

1.4 Pulse-Echo Principle

Diagnostic ultrasound operates on the pulse-echo principle:

1. A short pulse of ultrasound is transmitted into the body.
2. The pulse travels through tissues, encountering interfaces where partial reflection occurs.
3. Reflected echoes return to the transducer.
4. The time delay between transmission and reception is measured.
5. Distance = (c × time) / 2 (the factor of 2 accounts for the round trip).

Pulse Characteristics

• Pulse duration: The time from the beginning to the end of a single pulse. Typically 2–4 cycles. Shorter pulses give better axial resolution.
• Pulse repetition frequency (PRF): The number of pulses transmitted per second. Lower PRF at greater depths (need longer listening time).
• Duty factor: The fraction of time the transducer is transmitting. Typically <1% in diagnostic B-mode.
• Spatial pulse length (SPL): The physical length of a pulse in tissue. SPL = number of cycles × λ. Determines axial resolution.

Axial Resolution = SPL / 2

Two structures separated by less than half the SPL cannot be distinguished as separate.

1.5 Imaging Modes

A-Mode (Amplitude Mode)

The simplest form — echoes are displayed as vertical deflections on an oscilloscope screen. The amplitude of the deflection represents echo strength. Historical significance: Used in early echoencephalography (midline shift detection). Rarely used clinically today.

B-Mode (Brightness Mode)

The fundamental mode for modern diagnostic imaging. Echoes are displayed as dots of varying brightness (greyscale). Brighter dots = stronger echoes.

• 2D greyscale imaging — real-time cross-sectional anatomy
• Frame rate typically 15–30 Hz in obstetric imaging
• Each image frame is built from multiple scan lines (128–512 lines)

M-Mode (Motion Mode)

A single scan line is displayed over time. The x-axis is time, and the y-axis is depth. Used for: - Measuring fetal cardiac activity (6 weeks gestation) - Assessing fetal heart rhythms and wall motion - Measuring fetal cardiac dimensions - Assessing diaphragmatic motion

Doppler Modes

Covered in detail in Section 3.

1.6 Image Formation and Quality

Frame Rate

Frame rate = PRF / Number of scan lines per frame

Factors affecting frame rate: - Depth: Greater depth → longer listening time → lower PRF → lower frame rate - Line density: More lines → better lateral resolution but lower frame rate - Sector width: Wider sector → more lines → lower frame rate - Multi-focus zones: More focal zones → more pulses per line → lower frame rate

Clinical trade-off: In obstetric scanning, adequate frame rate (>15 Hz) is needed to detect fetal cardiac activity and movement. Transvaginal probes (shallow depth) naturally allow higher frame rates.

Resolution Types

Axial Resolution

• Definition: Ability to distinguish two structures lying parallel to the ultrasound beam (along the beam axis).
• Determined by: Spatial pulse length (SPL). Shorter SPL = better axial resolution.
• Typical value: 0.5–1.0 mm at 3.5 MHz
• Improvement: Higher frequency probes, shorter pulse duration (good damping)

Lateral Resolution

• Definition: Ability to distinguish two structures lying perpendicular to the beam.
• Determined by: Beam width at the focal zone.
• Typical value: 2–4 mm at 3.5 MHz
• Improvement: Proper focusing, appropriate focal zone placement

Elevational Resolution (Slice Thickness)

• Definition: Resolution in the plane perpendicular to the scan plane (the "third dimension").
• Determined by: Transducer geometry and focusing in the elevational plane.
• Typical value: 5–10 mm at typical depths
• Clinical issue: "Partial volume averaging" — structures within the slice thickness appear superimposed on the image.

Temporal Resolution

• Definition: The ability to accurately depict moving structures over time.
• Determined by: Frame rate.
• Clinical relevance: Important for fetal cardiac assessment and Doppler studies.

Gain and Time Gain Compensation (TGC)

• Overall gain: Uniform amplification of all returning echoes. Too high → image appears too bright ("washed out"); too low → image too dark.
• Time Gain Compensation (TGC): Also called depth gain compensation. Because deeper echoes are attenuated more, TGC applies progressively greater amplification to deeper signals to produce a uniformly bright image from top to bottom.
• TGC sliders: Typically 6–8 sliding controls that adjust gain at different depths.

Dynamic Range

• Definition: The range of echo amplitudes that the system can display, from the weakest to the strongest detectable signal.
• Measured in decibels (dB). Typical range: 40–70 dB.
• Low dynamic range: High contrast ("black and white" appearance) — useful for detecting subtle differences
• High dynamic range: Many shades of grey — smoother, more realistic appearance
• Clinical use: Cystic structures (fluid — anechoic) require different dynamic range settings than solid organs.

Focal Zone

• The region where the ultrasound beam is narrowest and lateral resolution is best.
• Single focus vs. multiple foci (multiple foci improve image uniformity but reduce frame rate).
• In obstetric scanning, place the focal zone at the level of the structure of interest (e.g., fetal heart, fetal head for BPD measurement).

1.7 Ultrasound Artefacts

Artefacts are features in the image that do not accurately represent the anatomical structures being examined. Recognising artefacts is crucial for correct interpretation.

Acoustic Shadowing

• Appearance: A dark (echo-free) region distal to a strongly reflective or attenuating structure.
• Cause: Near-total reflection or absorption of the ultrasound beam at a high-impedance interface (e.g., bone, gallstone, calcification, fetal ribs).
• Clinical relevance: Can obscure structures behind the shadowing object. However, it helps identify calcifications and stones.

Acoustic Enhancement (Posterior Enhancement / Through-Transmission)

• Appearance: A bright (echogenic) region distal to a fluid-filled structure.
• Cause: Fluid attenuates sound less than soft tissue. When sound passes through fluid, less attenuation occurs, so echoes from beyond the fluid are amplified compared to surrounding tissue at the same depth.
• Clinical relevance: Confirms cystic nature of a structure. Key sign in differentiating cysts from solid masses.

Edge Shadowing (Refraction Shadowing)

• Appearance: Thin, dark lines extending distally from the edges of a curved reflector (e.g., bladder, cyst, fetal skull).
• Cause: Refraction of the ultrasound beam at the curved interface — the beam is deviated, reducing echo return from that path.
• Clinical relevance: Can mimic pathology if not recognised.

Reverberation Artefact

• Appearance: Multiple equally spaced bright lines ("A-lines") parallel to the transducer face, diminishing in intensity with depth.
• Cause: Sound bounces back and forth between two highly reflective parallel surfaces (e.g., transducer face and a strong reflector like the bladder wall).
• Clinical relevance: Common in transvaginal scanning. Can be confused with pathology.

Mirror Image Artefact

• Appearance: A duplicate of an anatomical structure appears on the other side of a strongly reflective interface.
• Cause: The ultrasound beam reflects off a strong reflector (e.g., diaphragm) and interacts with a structure before returning along the same path, making the machine interpret the structure as being on the other side.
• Clinical relevance: Commonly seen at the diaphragm — liver may appear "in the chest." In obstetrics, may see a "twin" gestational sac in early pregnancy.

Comet Tail Artefact

• Appearance: A short, tapering trail of echoes distal to a small, highly reflective structure.
• Cause: A form of reverberation from closely spaced reflective interfaces (e.g., surgical clips, calcifications, cholesterol crystals).
• Clinical relevance: Distinguished from ring-down artefact (though the terms are sometimes used interchangeably).

Ring-Down Artefact

• Appearance: Solid, narrow vertical bands of continuous echoes distal to gas bubbles.
• Cause: Resonance within gas bubbles trapped between tissues, producing continuous sound emission.
• Clinical relevance: Seen distal to bowel gas. Can be confused with pathology.

Refraction Artefact

• Appearance: Misregistration of structures, appearing displaced from their true anatomical position.
• Cause: The ultrasound beam bends (refracts) as it passes between tissues of different propagation speeds.
• Clinical relevance: Can cause errors in measurement if the beam path is not straight.

Side Lobe and Grating Lobe Artefacts

• Appearance: Bright, often curvilinear echoes appearing in an anechoic region (e.g., within a cyst or bladder).
• Cause: The ultrasound beam has low-intensity "side lobes" that detect echoes from off-axis reflectors. The machine assumes these echoes came from the main beam direction and displays them incorrectly.
• Clinical relevance: Can mimic debris, sludge, or septations within a cyst.
• Reduction: Use of phased arrays and electronic focusing reduces side lobes.

Speed Error Artefact

• Appearance: Structures appear deeper or shallower than they truly are.
• Cause: The machine assumes a constant speed of 1540 m/s. In tissues with different speeds (e.g., fat = 1450 m/s), the time-of-flight calculation gives erroneous depth.
• Clinical relevance: Minor effect in most O&G imaging. More significant in breast imaging (fat) and musculoskeletal imaging.

Slice Thickness Artefact

• Appearance: Low-level echoes within a fluid-filled structure.
• Cause: The ultrasound beam has thickness in the elevational plane. Strong reflectors adjacent to the fluid-filled structure are included in the slice and displayed as if they were within the fluid.
• Clinical relevance: Can mimic debris, septations, or solid components in cysts.

1.8 Bioeffects and Safety of Ultrasound

ALARA Principle

As Low As Reasonably Achievable — the guiding principle for ultrasound safety. Use the lowest output power and shortest examination time necessary to obtain diagnostic information.

Thermal Index (TI)

A measure of the potential for tissue heating.

• TI = W / Wdeg where W is the acoustic power and Wdeg is the power required to raise tissue temperature by 1°C.
• TIB (Bone TI): Used when bone is at the focus — relevant in late pregnancy (fetal skull, spine)
• TIS (Soft Tissue TI): Used for soft tissue examinations
• TIC (Cranial TI): Used for transcranial examinations

BMUS (British Medical Ultrasound Society) Guidelines: - For obstetric ultrasound, keep TI ≤ 0.7 (≤0.3 for Doppler in first trimester) - For Doppler studies in first trimester, limit exposure time

Mechanical Index (MI)

A measure of the potential for non-thermal bioeffects, particularly cavitation (formation and collapse of gas bubbles).

MI = peak rarefactional pressure / √frequency

• MI < 0.3: No risk of cavitation
• MI 0.3–0.7: Theoretical risk
• MI > 0.7: Significant risk (avoid in ophthalmic applications)
• MI > 1.0: Significant risk — restrict exposure

Clinical guidance: Keep MI < 0.7 for obstetric ultrasound when possible. Highest MI occurs with colour/power Doppler and pulsed wave Doppler.

Safety Guidelines (BMUS)

Bioeffects of Ultrasound

Thermal effects: - Tissue heating from absorption of ultrasound energy - Fetal tissue is most sensitive to temperature rise - Temperature rise >1.5°C above baseline may cause fetal damage - Risk is highest with spectral Doppler (highest output power) - Risk is lowest with B-mode (lowest output power)

Mechanical effects: - Cavitation: Formation, oscillation, and collapse of gas bubbles - Stable cavitation: Microbubbles oscillate (harmless at diagnostic levels) - Inertial cavitation: Bubble collapse produces heat, free radicals, shock waves (potentially harmful) - Contrast microbubbles lower the threshold for cavitation

Epidemiological evidence: - No confirmed adverse fetal effects from diagnostic ultrasound in humans - Studies have shown no increase in childhood malignancies, congenital anomalies, or developmental delay - However, theoretical risks exist → ALARA principle is essential

2. Obstetric Ultrasound

2.1 First Trimester Ultrasound (Weeks 0–13+6)

Gestational Sac

• Visible from: ~4.5 weeks (transvaginal) / 5 weeks (transabdominal)
• Appearance: Anechoic (black) fluid collection within the echogenic endometrium
• Mean sac diameter (MSD): Average of three orthogonal measurements
• Gestational age (weeks) = MSD (mm) + 30
• By 5.5 weeks, the sac should measure ~10 mm
• The intradecidual sign is the earliest sign of intrauterine pregnancy — a small fluid collection surrounded by echogenic decidua
• The double decidual sac sign (two concentric echogenic rings around the gestational sac) helps differentiate true gestational sac from pseudosac (seen in ectopic pregnancy)

Yolk Sac

• Visible from: ~5 weeks (transvaginal)
• Appearance: Small (3–5 mm), round, anechoic structure with a bright rim, located within the gestational sac
• Function: Provides nutrients to the embryo before the placental circulation develops
• Normal size: 3–5 mm until ~10 weeks, then regresses
• Abnormal: Yolk sac >6 mm or <2 mm is associated with abnormal pregnancy outcome

Fetal Pole

• Visible from: ~5.5–6 weeks (transvaginal)
• Initially appears as a 1–2 mm echogenic linear structure adjacent to the yolk sac
• By 6.5 weeks, the fetal pole should be visible when the MSD is ≥15 mm (transvaginally)

Cardiac Activity

• Visible from: ~5.5–6 weeks
• Fetal heart rate (FHR) at 6 weeks: ~100–115 bpm
• FHR at 9 weeks: ~170–180 bpm (peak, then declines)
• FHR > 180 bpm in first trimester: associated with increased miscarriage risk
• FHR < 100 bpm at 6–7 weeks: poor prognosis
• Normal FHR progression:
• 6 weeks: 100–115 bpm
• 7 weeks: 120–145 bpm
• 8 weeks: 145–170 bpm
• 9 weeks: 155–185 bpm (peak)
• 12 weeks: ~150 bpm
• M-mode is the preferred technique for accurate FHR measurement (not spectral Doppler, which has higher thermal output)

Crown-Rump Length (CRL)

• The standard measurement for dating pregnancy between 7 and 13+6 weeks
• Also the most accurate dating method overall (±5–8 days)
• Should be measured in a mid-sagittal plane with the fetus in neutral position (not hyperextended or flexed)
• The biparietal diameter (BPD) is the next most accurate for dating after CRL

Formula: Gestational age (days) = CRL (mm) + 42 (Robinson's formula, simplified)

Important notes: - If CRL is <7 mm and no cardiac activity is seen → repeat scan in 7–14 days before diagnosing miscarriage - If CRL is ≥7 mm and no cardiac activity → definite miscarriage (NICE / RCOG guideline) - If MSD is ≥25 mm and no fetal pole → definite miscarriage - NICE recommends offering a repeat scan in 14 days for uncertain viability

Nuchal Translucency (NT)

• Measured at: 11+0 to 13+6 weeks (CRL 45–84 mm)
• Appearance: Anechoic fluid collection at the back of the fetal neck, between the skin and the soft tissue overlying the cervical spine
• Measurement technique (NT standardised technique):
• Mid-sagittal section of the fetus
• Neutral fetal head position (not extended or flexed)
• Calipers placed on the inner echo of the skin and the outer echo of the soft tissue over the spine
• Measure at the widest part of the translucency
• Image magnification such that the fetal head and thorax fill 75% of the screen
• Distinguish NT from amnion (which appears as a separate thin membrane)
• Normal: <3 mm (or <99th centile for CRL)
• Increased NT: >99th centile (typically ~3.5 mm) → risk of:
• Trisomy 21 (Down syndrome)
• Trisomy 13 (Patau) and 18 (Edwards)
• Turner syndrome (45,XO)
• Cardiac anomalies (in euploid fetuses)
• Other structural anomalies (diaphragmatic hernia, skeletal dysplasias)
• Noonan syndrome
• Congenital infection
• Combined screening (FMF / NICE): NT + maternal serum β-hCG + PAPP-A → detection rate ~85% for trisomy 21 at a 5% false-positive rate

Nasal Bone Assessment

• Appearance: Echogenic line (ossified) seen in the profile view at 11–13+6 weeks, parallel to and in front of the nasal skin
• Absent/hypoplastic nasal bone: Associated with trisomy 21 (~70% of Down fetuses have absent nasal bone at 11–13 weeks vs. 1–3% of euploid fetuses)
• Measurement technique: Mid-sagittal view with 45–90° angle, just below the skin line
• Used as part of the combined or sequential screening tests

Ductus Venosus Doppler

• Waveform: Triphasic pattern with S-wave (ventricular systole), D-wave (early diastole), and a-wave (atrial contraction)
• Normal: Forward flow throughout the cardiac cycle (positive a-wave)
• Abnormal: Reversed a-wave → associated with:
• Trisomy 21, 18, 13
• Cardiac defects
• Fetal compromise
• Used as a secondary screening marker in first-trimester combined screening (increased NT + reversed a-wave → high risk of trisomy 21)

Tricuspid Flow

• Nyquist limit: Adjusted to 30–40 cm/s for assessment
• Normal: Single forward flow peak (during ventricular systole)
• Tricuspid regurgitation (TR): Detected in 1–5% of euploid fetuses but 55–65% of fetuses with trisomy 21
• Assessed with colour Doppler and spectral Doppler (continuous wave)
• Used as an additional marker in first-trimester screening

2.2 Second Trimester Ultrasound (Weeks 14–27+6)

Dating Protocol

If a first-trimester CRL was not obtained, second-trimester biometric measurements are used. The hierarchy of accuracy is: 1. CRL (first trimester) — best 2. BPD — second best 3. HC (head circumference) 4. FL (femur length) 5. AC (abdominal circumference) — least accurate for dating

Hadlock charts are the most widely used reference for fetal biometry and weight estimation.

Fetal Biometry

Biparietal Diameter (BPD)

• Measurement plane: Transaxial view at the level of the thalami and cavum septum pellucidum
• The falx cerebri should be midline
• Calipers placed on the outer edge of the proximal skull and the inner edge of the distal skull (outer-to-inner)
• Alternatively, outer-to-outer measurement (consistency is important)
• Normal growth: Increases approximately 3–4 mm per week in the second trimester
• Important: BPD is reliable until ~28 weeks; after that, head shape variations (brachycephaly, dolichocephaly) may cause inaccuracy

Head Circumference (HC)

• Measured on the same image as BPD (thalami, cavum septum pellucidum, symmetrical hemispheres)
• Either directly traced around the outer perimeter of the skull, or calculated from BPD and occipitofrontal diameter (OFD):
• HC (calculated) = (BPD + OFD) × 1.57
• Less affected by head shape than BPD
• Used for dating in the second/third trimesters

Abdominal Circumference (AC)

• Measurement plane: Transverse view at the level of the umbilical vein (portal sinus) and stomach bubble
• The ribs should be visible (ensuring a true transverse cut, not oblique)
• The kidneys should NOT be visible (plane too low)
• The heart should NOT be visible (plane too high)
• Direct tracing around the outer perimeter of the skin is the most accurate method
• Alternatively, calculated from anteroposterior (AP) and transverse (TR) diameters:
• AC (calculated) = (AP + TR) × 1.57
• Clinical significance: AC is the best single predictor of fetal weight. AC lag suggests growth restriction or macrosomia.

Femur Length (FL)

• Measurement: The full ossified diaphysis of the femur (excluding the epiphyses and femoral head)
• The angle of insonation should be as close to 90° as possible
• Both ends of the ossified shaft should be clearly defined
• Normal growth: Increases ~2–3 mm per week
• Short femur (FL lagging): Consider short-limb skeletal dysplasia, trisomy 21, FGR

Other Measurements

• Humerus length: Similar to femur; used in some growth protocols
• Cerebellar diameter: Transverse cerebellar diameter increases ~1 mm per week up to ~20 weeks
• Nuchal fold: Measured at 18–22 weeks; thick nuchal fold >6 mm associated with trisomy 21
• Lateral ventricle (atrium): Measured at the level of the glomus of the choroid plexus; normal atrial width <10 mm (>10 mm = ventriculomegaly)
• Choroid plexus cysts: Present in ~1–2% of normal fetuses; associated with trisomy 18 if other markers present
• Echogenic intracardiac focus (EIF): Present in ~3–5% of normal fetuses; associated with trisomy 21 if other markers present
• Echogenic bowel: Brightness equal to or greater than surrounding bone; associated with trisomy 21, CF, FGR, congenital infection

Fetal Weight Estimation

Hadlock formula (most commonly used):

Log₁₀(EFW) = 1.326 − 0.00326(AC)(FL) + 0.0107(HC) + 0.0438(AC) + 0.158(FL)

Other formulas: Shepard, Warsof, Campbell.

Accuracy: Estimated fetal weight is accurate to within ±15% of actual birth weight in ~80% of cases.

Anomaly Scan (18–20+6 Weeks)

The mid-trimester fetal anomaly scan is a detailed anatomical survey offered to all pregnant women, typically between 18+0 and 20+6 weeks (NICE recommends 18+0–20+6 weeks).

Scan Protocol (Systematic Examination)

Head and Brain: - Skull shape and integrity (anencephaly, encephalocele) - Falx cerebri (midline) - Cavum septum pellucidum (present — absent in holoprosencephaly, agenesis of corpus callosum) - Thalami - Ventricles (atria <10 mm — excludes ventriculomegaly) - Choroid plexus (cysts — usually benign but may prompt karyotyping if other markers) - Cerebellum (normal shape — "banana sign" is absent in Dandy-Walker) - Cisterna magna (depth 2–10 mm) - Nuchal fold (<6 mm at 18–22 weeks)

Face: - Profile view (exclude severe micrognathia, abnormal nasal bone) - Lips (intact — exclude cleft lip/palate) - Orbits (present — exclude hypotelorism/hypertelorism)

Spine: - Longitudinal view — check curvature and intact skin contour - Transverse and coronal views — three ossification centres (vertebral body + two neural arches) - Exclude: spina bifida, sacral agenesis, vertebral anomalies

Chest: - Lung fields (symmetrical, homogeneous) - Diaphragm (intact — exclude diaphragmatic hernia) - Heart in left chest (dextrocardia suggests situs abnormality)

Heart (4-Chamber View + Outflow Tracts): The 4-chamber view detects ~60% of major cardiac anomalies. - Two atria of approximately equal size - Two ventricles of approximately equal size - Intact ventricular septum - Foramen ovale flap in the left atrium - Normal atrioventricular valve offset (tricuspid is more apical than mitral) - Heart occupies ~⅓ of the thoracic area

Outflow tracts: - Left ventricular outflow tract (aorta): arises from left ventricle, courses to the right - Right ventricular outflow tract (pulmonary artery): crosses over the aorta - Normally related great arteries: the pulmonary artery crosses anterior and to the left of the aorta

Abdomen: - Stomach (present in left upper quadrant — situs) - Kidneys (bilateral reniform shape, no significant hydronephrosis) - Bladder (visible) - Cord insertion (exclude omphalocele, gastroschisis — detected by 12 weeks) - Abdominal wall intact

Limbs: - Three segments of each limb (arms: humerus, radius/ulna, hands; legs: femur, tibia/fibula, feet) - Number of digits (feet: count toes; hands: count fingers usually by waving) - Position (clubfoot, rocker-bottom feet) - Complete long bone assessment

Placenta: - Position (anterior, posterior, fundal, low-lying, praevia) - Cord insertion (central, marginal, velamentous) - Texture (normal, not too thick) - Succenturiate lobe (abnormal additional placental lobes)

Cord: - Three-vessel cord (two arteries, one vein) - Single umbilical artery (SUA): associated with renal anomalies, FGR, aneuploidy

Amniotic Fluid: - Normal: subjective assessment (or AFI/ DVP) - Oligohydramnios: Reduced pockets - Polyhydramnios: Excess fluid

2.3 Third Trimester Ultrasound (Weeks 28–40+)

Growth Scans

Indicated when there are risk factors for: - Fetal growth restriction (FGR): Maternal hypertension, pre-eclampsia, diabetes, autoimmune disease, smoking, previous FGR, multiple pregnancy - Macrosomia: Maternal diabetes, obesity, post-dates, excessive weight gain - Multiple pregnancy: Serial growth scans every 2–4 weeks from 24 weeks

Amniotic Fluid Assessment

Amniotic Fluid Index (AFI)

• The uterus is divided into four quadrants using the linea nigra and a transverse line at the maternal umbilicus.
• The deepest vertical pocket in each quadrant (excluding fetal parts and cord) is measured.
• AFI = sum of the four deepest pockets.

Deepest Vertical Pocket (DVP)

• The single deepest vertical pocket of amniotic fluid (at least 1 cm wide) is measured.
• Normal: 2–8 cm
• Oligohydramnios: DVP <2 cm
• Polyhydramnios: DVP >8 cm

Controversy: The AFI method has a higher false-positive rate for oligohydramnios, leading to more inductions without improved outcomes. The DVP method is preferred by some authorities (NICE recommends DVP for liquor assessment).

Doppler Studies in Third Trimester

Covered in detail in Section 3.

Cervical Length Assessment

• Measurement: Transvaginal sonography (TVS) is the gold standard
• Empty bladder first
• Measure the distance between the internal os and external os along the endocervical canal
• Normal: >30 mm
• Short cervix:
• <25 mm at 16–24 weeks → increased risk of preterm birth
• <15 mm → very high risk
• <10 mm → extremely high risk
• Cervical length <25 mm at mid-trimester screening: consider progesterone, cervical cerclage (if history of preterm birth)
• Bishop score: Can be assessed by ultrasound (though traditionally clinical). Components: cervical length, funneling, position, consistency, station

2.4 Specialised Obstetric Ultrasound

Twin Pregnancy

Chorionicity Determination

Importance: Chorionicity determines management, surveillance intervals, and prognosis. Monochorionic twins have the highest risk.

Determined at 11–13+6 weeks (best accuracy):

Lambda sign (twin peak sign): Wedge-shaped placental tissue extending into the inter-twin membrane. This is the most reliable sign of DC twins (96–100% positive predictive value).

T-sign: The inter-twin membrane inserts directly into the placenta at a right angle without a wedge of placental tissue. Indicates MC twins.

After first trimester: The membrane becomes thinner and the lambda/T signs become less reliable.

Inter-twin membrane: - DC: Two layers of amnion + two layers of chorion (4 layers) - MC: Two layers of amnion (2 layers) — no intervening chorion

Twin-to-Twin Transfusion Syndrome (TTTS)

Incidence: 10–15% of monochorionic diamniotic (MCDA) twins

Pathophysiology: Unbalanced shunting of blood through placental arteriovenous anastomoses - Donor twin: Hypovolaemia, oliguric → oligohydramnios (stuck twin) - Recipient twin: Hypervolaemia, polyuric → polyhydramnios, cardiac strain

Quintero Staging System:

Management: - Stage I: Expectant management or amnioreduction - Stage II–IV: Fetoscopic laser ablation of placental anastomoses (Solomon technique) - Amnioreduction: Palliative — reduces polyhydramnios but does not address the underlying shunt - Laser therapy: Survival rate ~70–85% for at least one twin

Selective Fetal Growth Restriction (sFGR) in Twins

• Definition: Discordance in estimated fetal weight ≥25% in MC twins or abdominal circumference <10th centile in one twin
• Types based on umbilical artery Doppler:
• Type I: Positive end-diastolic flow (UAEDF present) — better prognosis
• Type II: Cyclic/recurrent absent end-diastolic flow (AEDF)
• Type III: Persistent AEDF/REDF — worst prognosis

Twin Anaemia-Polycythaemia Sequence (TAPS)

• Chronic inter-twin transfusion through small arteriovenous anastomoses
• Diagnosis: MCA PSV discordance — donor (anaemic) MCA PSV >1.5 MoM; recipient (polycythaemic) MCA PSV <1.0 MoM
• Staging: Based on MCA PSV values (Stage I–V)
• Difference from TTTS: No oligohydramnios/polyhydramnios sequence (amniotic fluid volumes are normal)

Placental Location

• Determined at the mid-trimester anomaly scan
• Low-lying placenta: Extending over or within 20 mm of the internal cervical os (NICE 2018)
• Placenta praevia: Covers the os completely or partially
• Minor (Type I/II): Does not cover the os (low-lying)
• Major (Type III/IV): Covers the os
• Vasa praevia: Fetal blood vessels crossing the internal os. Diagnosed by colour Doppler. High fetal mortality if unrecognised. Associated with velamentous cord insertion and bilobed/succenturiate placenta.
• Placental migration: The lower segment elongates in the third trimester, so low-lying placentae at 20 weeks may appear away from the os by term (~90% resolve).

Cord Insertion

• Central: Cord inserts into the centre of the placental mass
• Marginal (Battledore): Cord inserts at the edge of the placenta
• Velamentous: Cord inserts into the membranes, with vessels traversing the membranes to the placenta. Associated with FGR, vasa praevia, and adverse outcomes.

Biophysical Profile (BPP)

Covered in detail in Section 7.

3. Doppler Ultrasound

3.1 Principles of Doppler Ultrasound

The Doppler Effect

When a sound wave is reflected from a moving target, the frequency of the reflected wave is shifted. This is the Doppler shift (Δf).

Δf = 2 × f₀ × v × cos θ / c

Where: - Δf = Doppler shift (Hz) - f₀ = transmitted ultrasound frequency (Hz) - v = velocity of the moving target (m/s) — typically red blood cells - θ = angle between the ultrasound beam and the direction of flow (degrees) - c = speed of sound in tissue (~1540 m/s)

Key Clinical Implications

• Angle dependence: The Doppler shift depends on cos θ. At θ = 0° (beam parallel to flow), cos θ = 1 → maximum shift. At θ = 90° (beam perpendicular to flow), cos θ = 0 → no Doppler shift detected.
• Critical angle: Angle correction must be applied when θ > 0°. If θ > 60°, even small measurement errors in θ cause large errors in velocity calculation → measurements become unreliable.
• Velocity calculation: v = Δf × c / (2 × f₀ × cos θ)

Angle of Insonation

• Ideal: <60°
• Acceptable: 30–60° — angle correction applied
• Unreliable: >60° — avoid quantitative velocity measurements
• Cannot measure: 90° — no Doppler shift detected

Doppler Shift Direction

• Positive shift (flow towards transducer): Displayed as upward deflection (above baseline)
• Negative shift (flow away from transducer): Displayed as downward deflection (below baseline)
• Baseline adjustment: Allows display of flows of different directions; can be shifted to optimise the display of unidirectional flow patterns

3.2 Types of Doppler

Continuous Wave (CW) Doppler

• Uses two separate crystals: One continuously transmits, one continuously receives
• No depth discrimination — all signals along the beam path are recorded simultaneously
• Advantages:
• Can measure very high velocities (no aliasing)
• Simple and inexpensive
• High sensitivity
• Disadvantages:
• Range ambiguity — cannot determine where along the beam the signal originates
• No spatial resolution
• Uses: Fetal echocardiography (high-velocity jets across stenotic/regurgitant valves), peripheral vascular studies
• Nyquist limit: Not applicable (no PRF — continuous transmission)

Pulsed Wave (PW) Doppler

• Uses a single crystal: Alternately transmits and receives pulses (same as B-mode)
• Sampling depth is selected by a range gate (sample volume)
• Advantages:
• Range resolution — can sample flow from a specific vessel or location
• Allows simultaneous anatomical imaging (duplex) and colour flow mapping (triplex)
• Disadvantages:
• Aliasing — limited by the Nyquist limit (PRF / 2): Δf ≤ PRF / 2
• Cannot measure very high velocities without using a high PRF (HPRF) mode
• Nyquist limit: The maximum Doppler shift that can be measured unambiguously. If the Doppler shift exceeds PRF/2, aliasing occurs (the waveform wraps around the baseline).

Colour Doppler

• Colour-codes flow information superimposed on a B-mode image
• Direction: Traditionally red = towards transducer, blue = away from transducer (BART: Blue Away, Red Towards — but colour maps can be inverted)
• Velocity: Lighter shades = higher velocity
• Variance/Turbulence: Green/yellow mosaic pattern indicates turbulent flow
• Based on autocorrelation of multiple PW Doppler signals from each pixel
• Advantages: Visualises flow patterns rapidly, aids vessel identification
• Disadvantages: Lower frame rate, angle-dependent, subject to aliasing, less sensitive to low velocities

Power Doppler (Angio/Energy Doppler)

• Displays the amplitude (energy) of the Doppler signal, not the frequency shift
• Advantages:
• Much more sensitive to low-velocity/low-volume flow
• Not angle-dependent (detects flow even at 90°)
• No aliasing
• Better delineation of vessel continuity and organ perfusion
• Disadvantages:
• No directional information (all flow appears as one colour)
• No velocity estimation
• Very sensitive to motion artefact (flash artefact)
• Uses: Ovarian and endometrial blood flow assessment, placental vascularity, tumour vascularity, testicular flow

Spectral Doppler

Displays the complete range of velocities within the sample volume over time: - X-axis: Time - Y-axis: Velocity (cm/s or kHz shift) - Brightness: Amplitude of signal at each velocity (number of red blood cells moving at that velocity)

Waveform analysis: - Arterial: Pulsatile waveform with systolic peak and diastolic trough - Venous: Continuous flow, may have respiratory variation - High-resistance waveform: Low diastolic flow (e.g., umbilical artery in FGR) - Low-resistance waveform: High diastolic flow (e.g., internal carotid artery, uterine artery after 26 weeks)

3.3 Doppler Indices

Resistive Index (RI) — Pourcelot Index

RI = (PSV − EDV) / PSV

• Range: 0 (continuous forward flow) to 1 (no end-diastolic flow)
• High RI: Increased downstream resistance
• Low RI: Low downstream resistance
• Normal umbilical artery RI: Decreases with gestational age (as the placenta matures)
• Use: May be less angle-dependent than PI in some settings

Pulsatility Index (PI) — Gosling Index

PI = (PSV − EDV) / Mean Velocity (MV)

• More robust than RI because it uses mean velocity
• Preferred index in most obstetric Doppler applications (uterine artery, umbilical artery, MCA)
• Normal umbilical artery PI: Decreases with gestational age (from ~1.5 at 20 weeks to ~0.8 at 40 weeks)
• PI > 95th centile: Abnormal (increased placental resistance)
• PI < 5th centile: Abnormal (low resistance, e.g., brain-sparing in MCA)

Systolic-Diastolic Ratio (S/D Ratio)

S/D Ratio = PSV / EDV

• Simple but non-standardised (extreme values with low EDV)
• Less commonly used now (PI is preferred)

Peak Systolic Velocity (PSV)

• The maximum velocity during ventricular systole
• MCA PSV: Used to detect fetal anaemia (PSV > 1.5 MoM for gestational age)
• Uterine artery PSV: Left/right differences may indicate pathology
• Ductus venosus PSV: Reflects atrial pressure

End-Diastolic Velocity (EDV)

• The velocity at the end of diastole
• Reduced EDV: High downstream resistance
• Absent EDV (AEDF): Severe pathology — seen in severe FGR
• Reversed EDV (REDF): Very severe pathology — ominous sign

3.4 Specific Doppler Applications in Obstetrics

Uterine Artery Doppler

• Assessment window: 20–24 weeks (screening for pre-eclampsia and FGR)
• Technique: Transabdominal, lateral approach, identify the uterine artery at the apparent crossing with the external iliac artery
• Normal waveform: High-velocity, low-resistance waveform with continuous forward flow in diastole after ~26 weeks

Parameters assessed: - PI: Uterine artery PI decreases with gestation. Elevated PI (>95th centile) indicates abnormal trophoblast invasion - Notching: Early diastolic notch is normal in non-pregnant uterus and early pregnancy. Persistent diastolic notch after 24 weeks is abnormal - Bilateral vs unilateral: Bilateral notching or bilateral high PI confers higher risk than unilateral

Clinical significance: - High PI + bilateral notching at 20–24 weeks: - PPV for pre-eclampsia: ~25–40% (low PPV but high NPV) - PPV for FGR: ~20–30% - Negative predictive value: 95%+ (normal Doppler makes later development of pre-eclampsia/FGR unlikely) - Used in first-trimester combined screening for pre-eclampsia (uterine artery PI + PAPP-A + MAP + risk factors)

Umbilical Artery Doppler

• Assessment: Free loop of cord, sample volume over the whole lumen
• Normal: Forward flow throughout diastole, continuous flow

Grading of umbilical artery Doppler abnormality:

PERFORMANCE OF UA DOPPLER IN FGR: - UA PI is the first Doppler parameter to become abnormal in placental insufficiency - Sequential deterioration: UA PI ↑ → AEDF → REDF → DV a-wave reversal → fetal demise - RCT evidence: Use of UA Doppler in high-risk pregnancies reduces perinatal mortality by ~30% (GRIT trial, TRUFFLE trial)

Contraindications to UA Doppler (or used with caution): - Suspected fetal distress (may increase risk by unnecessary intervention) - Not recommended for low-risk screening

Middle Cerebral Artery (MCA) Doppler

MCA PSV for Fetal Anaemia

• Indication: Suspected fetal anaemia (red cell alloimmunisation, parvovirus B19, fetomaternal haemorrhage, TTTS, TAPS)
• Technique: Axial plane through the circle of Willis, angle correction <30°
• Normal: MCA PSV increases with gestational age (from ~20 cm/s at 20 weeks to ~60 cm/s at 40 weeks)
• Abnormal: MCA PSV > 1.5 MoM → moderate-to-severe fetal anaemia

Accuracy: MCA PSV >1.5 MoM has 100% sensitivity and 88% specificity for moderate-to-severe anaemia (Mari et al., 2000 — landmark paper).

Limitations: - Less reliable before 18 weeks or after 35 weeks - Affected by maternal position, fetal breathing, fetal heart rate - False positives: FGR with brain sparing, macrosomia, maternal fever

MCA PI for "Brain Sparing" Effect

• Normal: MCA PI decreases with gestational age
• Brain sparing: In chronic hypoxia, the fetal cerebral circulation dilates (vasodilatation of MCA) to maintain flow to the brain → decreased MCA PI
• MCA PI < 5th centile for gestational age → brain sparing (compensatory response to hypoxia)
• A low MCA PI is not pathognomonic — it reflects a compensatory mechanism

Ductus Venosus (DV) Doppler

• Anatomy: Small venous vessel connecting the intra-abdominal umbilical vein to the inferior vena cava (bypasses the liver)
• Waveform: Biphasic (S-wave: ventricular systole, D-wave: early diastole) with forward a-wave (atrial contraction)
• Assessment: Quantitative analysis of peak velocities, PI, and qualitative (forward, absent, reversed a-wave)

Clinical significance in FGR:

DV Doppler as a sign of fetal compromise: - After UA AEDF/REDF, the next Doppler parameter to worsen is the DV - DV a-wave reversal is a late sign — associated with acidaemia and impending fetal demise - The TRUFFLE trial showed that delivery based on DV changes (rather than CTG changes alone) may improve neurodevelopmental outcomes

In TTTS: Reversed a-wave in recipient's DV is a criterion for Quintero Stage III

Cerebral-Placental Ratio (CPR)

CPR = MCA PI / UA PI

• Normal: >1.0 (or >5th centile for gestational age)
• Abnormal: <1.0 (or <5th centile) → CPR < 1 indicates brain-sparing
• CPR is a more sensitive predictor of adverse outcome than either MCA PI or UA PI alone
• CPR < 1 predicts:
• Adverse perinatal outcome in suspected FGR
• Stillbirth (even in appropriately-grown fetuses)
• Caesarean section for fetal distress
• Neonatal ICU admission

Fetal Aorta Doppler

• Descending aorta: Can be measured in the thorax or abdomen
• Used less commonly than UA and MCA Doppler
• Thoracic aorta PI > 95th centile: Suggests increased afterload/placental resistance

3.5 Clinical Applications and Guidelines

Fetal Growth Restriction (FGR) — Doppler Surveillance

The typical Doppler progression in FGR:

Stage 1: Elevated UA PI
Stage 2: MCA brain-sparing (low MCA PI, low CPR)
Stage 3: UA AEDF → REDF
Stage 4: DV a-wave absent → reversed
Stage 5: Umbilical vein pulsations (also DV deterioration)
Stage 6: Abnormal CTG → delivery

Management implications (RCOG, ISUOG guidelines):

Fetal Doppler Recommendations

ISUOG (International Society of Ultrasound in Obstetrics and Gynaecology) Practice Guidelines:

• Umbilical artery Doppler is recommended in all cases of suspected FGR (Grade A evidence)
• MCA Doppler is recommended in FGR surveillance (Grade B evidence)
• DV Doppler is recommended for timing of delivery in early-onset FGR (<32 weeks)
• Uterine artery Doppler at 20–24 weeks for pre-eclampsia screening (Grade A for high-risk women)
• MCA PSV for anaemia detection (Grade A)
• Not recommended for low-risk screening in routine practice

RCOG Green-Top Guideline (No. 31 — The Investigation and Management of the Small-for-Gestational-Age Fetus):

• Serial UA Doppler measurements are recommended for all singleton pregnancies with suspected FGR
• MCA and DV Doppler are useful for monitoring early-onset FGR and timing delivery
• CPR should be assessed when MCA and UA Doppler are performed
• Doppler surveillance should be performed at 1–2 weekly intervals in FGR

Fetal Anaemia — Intrauterine Transfusion (IUT)

Indications for IUT: - MCA PSV > 1.5 MoM and fetal haematocrit <25–30% (or Hb deficit >2 SD below mean) - Confirmed by cordocentesis

Procedural considerations: - Procedure performed under ultrasound guidance - Cordocentesis at the placental cord insertion (or free loop/fetal intrahepatic vein) - Sedation with fetal neuromuscular blockade (pancuronium, atracurium, vecuronium) - Donor blood: O-negative, CMV-negative, irradiated, cross-matched with maternal serum, packed to haematocrit ~75–85% - Volume calculation: estimated fetoplacental blood volume × target Hb increase / donor Hb - Target post-transfusion Hb: ~45–50 g/L (or ~0.75 of mean for gestational age) - Repeat MCA PSV in 1–2 weeks; repeat IUT when PSV > 1.5 MoM again

Complications: - Procedure-related fetal loss: ~1–3% per procedure - Infection, preterm labour, cord haematoma, fetomaternal haemorrhage, exsanguination, bradycardia

4. Cardiotocography (CTG)

4.1 Principles of CTG

Cardiotocography simultaneously records fetal heart rate (FHR) and uterine contractions (tocography). The external transducer uses Doppler ultrasound for FHR and a pressure transducer (tocodynamometer) for contractions.

Equipment

External (standard): - FHR transducer: Ultrasound Doppler probe placed over the fetal back (optimal signal) - Tocodynamometer: Pressure-sensitive device placed over the uterine fundus - Limitations: Maternal/fetal movement artefacts, signal dropout, obese habitus, inaccurate contraction quantification

Internal (invasive): - Fetal scalp electrode (FSE): Screw-on spiral electrode attached to fetal scalp (requires ruptured membranes and ≥2 cm cervical dilatation) - Provides accurate beat-to-beat FHR (not available with external Doppler) - Detects each R-wave of the fetal ECG → instantaneous heart rate calculation - Contraindications: Active maternal herpes, HIV (relative contraindication), face/brow presentation, suspected fetal bleeding disorder - Intrauterine pressure catheter (IUPC): Measures contraction intensity in Montevideo units (MVUs) - Normal labour: 120–200 MVUs - Adequate contractions: 200–240 MVUs every 10 minutes - Tachysystole: >5 contractions in 10 minutes, or >200 MVUs in a 30-minute window

Fetal Heart Rate Parameters

Baseline FHR

• Definition: The mean FHR over a 5–10 minute window, excluding accelerations, decelerations, and periods of marked variability
• Normal (normocardia): 110–160 bpm (NICE 2022, FIGO 2015)
• Bradycardia (<110 bpm):
• Mild (100–109 bpm): Often benign (e.g., occipito-posterior position, maternal supine hypotension)
• Moderate (80–100 bpm): Investigate cause (cord compression, placental insufficiency, maternal hypoglycaemia, fetal heart block)
• Severe (<80 bpm): Fetal distress, urgent delivery required
• Causes of fetal bradycardia: Prolonged deceleration, cord prolapse, placental abruption, uterine hyperstimulation, maternal seizures, intrauterine infection, fetal congenital heart block (maternal anti-Ro/La antibodies)
• Tachycardia (>160 bpm):
• Mild (161–180 bpm): Maternal fever, chorioamnionitis, dehydration, thyrotoxicosis, drugs (β-sympathomimetics), preterm fetus (higher baseline)
• Severe (>180 bpm): Severe chorioamnionitis, fetal hypoxia (late sign), fetal tachyarrhythmia (SVT, atrial flutter)
• Note: Tachycardia alone (with normal variability and no decelerations) is rarely a sign of fetal acidosis

Baseline Variability

• Definition: Beat-to-beat changes in the baseline FHR due to the interaction between the sympathetic (accelerator) and parasympathetic (decelerator) nervous systems
• Measured as: The amplitude of fluctuation from peak to trough in bpm
• Categories:

Clinical significance: - Loss of variability is the most important single predictor of fetal acidaemia (more important than decelerations) - If variability is normal (5–25 bpm), fetal acidosis is unlikely (NPV ~95%) - Reduced variability + decelerations → high risk of acidosis → consider FBS or delivery

Accelerations

• Definition: A transient increase in FHR above the baseline
• Gestational age-specific criteria:
• ≥32 weeks: ≥15 bpm lasting ≥15 seconds (from onset to return to baseline)
• <32 weeks: ≥10 bpm lasting ≥10 seconds
• Clinical significance: The presence of accelerations in the intrapartum period is a reassuring sign of fetal well-being (indicates an intact CNS and autonomic function)
• Prolonged acceleration: >2 minutes but <10 minutes (>10 minutes = baseline change)
• Absent accelerations in labour: May occur in normal labour (especially in the second stage), but their presence is reassuring

Decelerations

Decelerations are transient decreases in FHR from the baseline.

Early Decelerations

Variable Decelerations

Variable decelerations: severity grading

Clinical significance: - Moderate variable decelerations: Very common in labour (up to 50% of all labours). Usually benign if variability is normal. - Severe, repetitive variable decelerations: May indicate significant cord compression. If associated with reduced variability, consider FBS or delivery. - Complicated variables: Variable deceleration with a late component → worse prognosis

Late Decelerations

Late decelerations are classified as: - Reflex late deceleration: Normal baseline variability present (fetus is hypoxic but not yet acidaemic) — may respond to intrauterine resuscitation - Non-reflex (ominous) late deceleration: Absent/reduced variability + late decelerations = fetal acidosis — urgent delivery required

Prolonged Deceleration

• Definition: A deceleration lasting >90 seconds but <3 minutes
• Causes: Umbilical cord prolapse, maternal seizures, maternal hypotension (epidural-related), epidural top-up, uterine hyperstimulation, placental abruption, maternal Valsalva, fetal head examination
• Management: Immediate intrauterine resuscitation + evaluation + preparation for emergency delivery

• 50% of prolonged decelerations resolve spontaneously within 3 minutes

• If >3 minutes: Fetal bradycardia — treat as an emergency

Sinusoidal Pattern

Causes of sinusoidal pattern: - Fetal anaemia (red cell alloimmunisation, fetomaternal haemorrhage, vasa praevia, twin-twin transfusion) - Fetal hypoxia/acidosis - Maternal narcotic administration (morphine, pethidine, nalbuphine) - Fetal administration of atropine - Chorioamnionitis - Fetal heart failure

Distinction from "pseudo-sinusoidal" pattern: - Pseudo-sinusoidal: Irregular waveform, shorter duration (<10 min), presence of accelerations, normal variability — benign (often due to fetal thumb-sucking) - True sinusoidal: Regular sine wave, absent accelerations, absent variability — pathological

4.2 CTG Classification Systems

NICE 2022 (Updated) / FIGO 2015 Classification

Both use a 3-tier classification system:

Overall classification (FIGO 2015):

FIGO 2015 Detailed Classification

Normal CTG (all features): - Baseline 110–160 bpm - Variability 5–25 bpm - No decelerations (or early decelerations)

Suspicious CTG (one or more features): - Baseline: 100–109 bpm or 161–180 bpm - Variability: <5 bpm for 30–50 min (not >50 min) - Variable deceleration present but not severe, and not >50% of contractions

Pathological CTG (features indicating fetal hypoxia): - Baseline <100 bpm or >180 bpm - Variability: <5 bpm for >50 min, or >25 bpm, or sinusoidal pattern - Repeated severe variable decelerations - Late decelerations - Prolonged deceleration

4.3 ST Analysis (STAN)

STAN technology analyses the ST segment of the fetal ECG (obtained from a fetal scalp electrode).

Key Parameters

• T/QRS ratio: The ratio of the T-wave amplitude to QRS amplitude
• Normal: T/QRS <0.25
• Elevated: T/QRS ≥0.25 (suggests cardiac response to hypoxia)
• ST events: Significant changes in the ST segment (elevated T/QRS, biphasic ST, ST depression)
• Episodes: Duration of ST changes

Physiological Basis

• Under hypoxia, fetal myocardium utilises anaerobic metabolism → lactate accumulation
• The myocardium releases intracellular potassium (via ATP-sensitive K⁺ channels) → prolongs repolarisation → elevated T-wave → elevated T/QRS
• This is a compensatory mechanism — a sign of myocardial adaptation to hypoxia, NOT acidosis

STAN Clinical Use

• STAN is used in conjunction with CTG
• A STAN alert (significant ST event) combined with a pathological CTG → indicates fetal hypoxia → recommend delivery
• No ST event + pathological CTG → may still be pathological (especially in early labour)
• FIGO guidelines: The combination of CTG + STAN reduces operative deliveries for fetal distress and metabolic acidosis at birth

Limitations

• Requires FSE (ruptured membranes, ≥2 cm dilated)
• Cannot be used in multiple pregnancy (cannot determine which fetus)
• Contraindicated in maternal infection (herpes, HIV)
• Not useful if the fetus is very premature (<34 weeks) — immature myocardium may not mount a ST response

4.4 Management of Abnormal CTG

Intrauterine Resuscitation (Conservative Measures)

When a pathological CTG pattern is identified:

1. Maternal position change: Left lateral position (reduces aortocaval compression, improves uteroplacental perfusion)
2. IV fluids: Bolus 500–1000 mL of crystalloid (corrects hypotension)
3. Oxygen: High-flow facemask O₂ at 10–15 L/min (improves maternal and fetal oxygenation)
4. Stop oxytocin: Discontinue or reduce oxytocin infusion (reduces uterine hyperstimulation)
5. Tocolysis: Terbutaline 0.25 mg SC or sublingual GTN or IV salbutamol (relaxes the uterus, improves fetal oxygenation in hyperstimulation)
6. Correct maternal hypotension: If epidural-related, give IV fluids, reduce epidural rate
7. Cord prolapse: If suspected, perform vaginal examination, elevate the presenting part
8. Fetal scalp stimulation: Digital stimulation or vibroacoustic stimulation (if acceleration produced, suggests no acidosis)

Fetal Scalp Blood Sampling (FBS)

Indications: - Pathological CTG pattern (FIGO 2015) or suspicious CTG with risk factors - Preferably performed in active labour (≥3–4 cm dilatation)

Contraindications: - Maternal infection (herpes, HIV, hepatitis B/C — relative) - Fetal bleeding disorder (suspected haemophilia, thrombocytopenia) - Face/brow presentation - Prematurity <34 weeks (relative)

Procedure: - Amnioscope passed through the cervix, displacing the membranes - Fetal scalp cleaned and sprayed with ethyl chloride (vasoconstriction → capillary blood) - Small lancet stab (2 mm depth) - Blood collected in heparinised capillary tube - Analyse pH within 5 minutes (on a blood gas analyser)

FBS Interpretation:

Important notes: - The false-positive rate for FBS is significant (fetal scalp oedema, maternal acidosis, prolonged second stage can lower pH) - FBS may need to be repeated if CTG remains pathological - FBS is not always available (requires trained staff, equipment) → many units now rely on CTG + STAN + clinical assessment - An ascending (rising) pH is reassuring regardless of the absolute value

Decision-to-Delivery Interval

• Category 1 (immediate threat to life): Decision-to-delivery interval <30 minutes (e.g., cord prolapse, uterine rupture, massive abruption, pathological CTG with FSE STAN event)
• Category 2 (maternal or fetal compromise): Decision-to-delivery interval <75 minutes
• The standard for "urgent" caesarean section: 30 minutes from decision to delivery (NICE, RCOG)

Cord Blood Gas Analysis

• Umbilical artery pH: Reflects fetal acid-base status at birth
• Normal: >7.20
• Acidosis: <7.20 (mild), <7.10 (moderate), <7.00 (severe — associated with neonatal encephalopathy)
• Umbilical artery base deficit:
• Normal: <8 mmol/L
• Severe: >12 mmol/L (increased risk of neonatal morbidity)
• Arterial vs. venous: UA pH is lower than UV pH (usually by 0.05–0.10 units); a narrow gap suggests poor perfusion

5. Radiology & Radiation Safety

5.1 X-Ray Production

Physics of X-Ray Generation

X-rays are produced when high-speed electrons collide with a metal target (usually tungsten) in an X-ray tube.

Two mechanisms:

Bremsstrahlung Radiation ("Braking Radiation")

• An incident electron passes near the nucleus of a tungsten atom
• The electron is deflected and decelerated by the electromagnetic field of the nucleus
• The lost kinetic energy is emitted as an X-ray photon
• Produces a continuous spectrum of X-ray energies (up to the maximum tube voltage kVp)
• Most X-rays in diagnostic radiology are produced via bremsstrahlung

Characteristic Radiation

• An incident electron ejects an inner-shell (K-shell) electron from a tungsten atom
• An outer-shell electron drops into the vacancy, emitting an X-ray photon with energy equal to the difference in binding energies
• Produces discrete energy peaks characteristic of the target material (tungsten: 69.5 keV for K-shell)
• Accounts for ~10–20% of X-rays in a diagnostic beam

X-Ray Tube Components

1. Cathode: A heated filament (tungsten) that thermionically emits electrons
2. Anode: A rotating tungsten disc that the electrons strike
3. Rotating anode (3000–10,000 rpm) dissipates heat better than fixed anode
4. Target angle: 7–20° from the horizontal (determines focal spot size)
5. Focal spot: Where the electron beam hits the anode
6. Small focal spot (~0.6 mm): Better resolution, less heat capacity
7. Large focal spot (~1.2 mm): Worse resolution, more heat capacity
8. Housing: Lead-shielded enclosure with an X-ray port (window)
9. Collimator: Adjustable lead shutters that limit the X-ray beam to the area of interest
10. Filtration: Aluminium or copper filters that absorb low-energy X-rays (which contribute to patient dose but not diagnostic image quality)
11. Grid: A series of lead strips that reduce scatter radiation reaching the detector (improves image contrast)

X-Ray Parameters

5.2 Imaging Modalities

Plain Film Radiography (X-Ray)

• Principle: Differential attenuation of X-rays by tissues of different densities
• Five radiographic densities (from black to white):
• Air (black)
• Fat (dark grey)
• Soft tissue/water (grey)
• Bone/calcium (light grey/white)

• Metal/contrast (bright white)

• Applications in O&G: Very limited due to ionising radiation

• Chest X-ray in pregnancy (with abdominal shielding) — e.g., suspected TB, pneumonia
• Abdominal X-ray — no longer routinely used (MRI/US preferred)
• X-ray pelvimetry — outdated
• X-ray for suspected maternal bowel obstruction, renal calculi (but US/CT preferred)
• Neonatal chest X-ray

Fluoroscopy

• Principle: Real-time X-ray imaging using an image intensifier or digital flat-panel detector
• Components: X-ray tube, image intensifier/ flat-panel, video camera, display monitor
• Applications in gynaecology:
• Hysterosalpingography (HSG) — X-ray contrast study of the uterine cavity and fallopian tubes
• Barium studies (if indicated)
• Dose considerations: Higher effective dose than plain film due to continuous exposure
• Pulse fluoroscopy: Reduces dose by pulsing the X-ray beam (typically 7.5–15 pulses/sec vs continuous)

Computed Tomography (CT)

• Principle: An X-ray tube rotates around the patient, acquiring multiple projection images; a computer reconstructs cross-sectional images
• Hounsfield units (HU):
• Air: −1000 HU
• Fat: −80 to −120 HU
• Water: 0 HU
• Soft tissue: +20 to +80 HU
• Bone: +400 to +1000+ HU
• Contrast-enhanced structures: variable (up to +300 HU)
• Window/level: Adjusting the greyscale mapping to visualise different tissues
• Lung window (wide): −500 to +1500 HU
• Bone window (wide): 0 to +1000+ HU
• Soft tissue window (narrow): −50 to +150 HU
• Contrast: Iodinated contrast agents (IV, oral, or rectal for abdominal/pelvic CT)
• Iodine has high atomic number → high attenuation → bright on CT
• Can cause contrast-induced nephropathy (CIN) — risk in pre-existing renal impairment
• Radiation dose:
• CTDIvol (CT dose index): The dose within a single slice (mGy)
• DLP (dose-length product): CTDIvol × scan length (mGy·cm)
• Effective dose: DLP × k (conversion factor) (mSv)
  • Pelvis CT: ~6–10 mSv
  • Abdomen/pelvis CT: ~8–14 mSv
  • Head CT: ~2 mSv
  • Chest CT: ~5–7 mSv

Magnetic Resonance Imaging (MRI)

• Principle: Uses strong magnetic field (1.5–3 T) and radiofrequency pulses to image protons (hydrogen) in tissues
• No ionising radiation — major advantage in pregnancy
• Key sequences:
• T1-weighted: Fat is bright, fluid is dark. Good for anatomy, fat-containing lesions
• T2-weighted: Fluid is bright, fat is intermediate. Good for pathology (oedema, inflammation, cysts, tumours)
• FLAIR (Fluid-Attenuated Inversion Recovery): Suppresses CSF signal — useful for brain imaging
• DWI (Diffusion-Weighted Imaging): Measures water diffusion — helpful for detecting acute stroke, tumours
• STIR (Short Tau Inversion Recovery): Fat suppression
• GRE (Gradient Echo): Sensitive to haemorrhage/iron deposition
• Applications in O&G:
• Placental MRI (placenta accreta spectrum, praevia)
• Fetal brain MRI (ventriculomegaly, corpus callosum anomalies, posterior fossa)
• Pelvic masses in pregnancy (differentiating fibroids from ovarian masses)
• Gynaecological malignancies (staging endometrial, cervical, ovarian cancer)
• Pelvic endometriosis evaluation
• Time of flight (TOF) MRA: Non-contrast MR angiography
• Gadolinium contrast:
• Crosses the placenta (excreted into amniotic fluid)
• FDA class C: Avoid in pregnancy unless absolutely essential (risk of nephrogenic systemic fibrosis in mother with renal impairment; unknown fetal effects)
• Use only when diagnostic information cannot be obtained by other means

MRI Safety

MRI contraindications: - Absolute: Cardiac pacemaker (some MR-conditional pacemakers now exist), cochlear implant, ocular metallic foreign body, implanted drug pump, ferromagnetic aneurysm clips, non-MR-conditional implants - Relative: Severe claustrophobia, morbid obesity (bore size limitations), inability to lie still, first trimester (theoretical risk — magnetic fields may affect developing embryos? No proven harm, but avoid unless essential)

5.3 Radiation Dosimetry

Physical Quantities

Tissue weighting factors (ICRP): - Gonads: 0.08 - Bone marrow: 0.12 - Breast: 0.12 - Lung: 0.12 - Thyroid: 0.04 - Bone surface: 0.01 - Skin: 0.01 - Stomach: 0.12 - Colon: 0.12 - Liver: 0.04 - Oesophagus: 0.04 - Bladder: 0.04 - Brain: 0.01 - Kidneys: 0.01 - Salivary glands: 0.01 - Remainder: 0.12

Effective Dose from Common Examinations

Deterministic vs Stochastic Effects

Deterministic Effects

• Threshold dose: A minimum dose must be exceeded for the effect to occur
• Severity increases with dose
• Examples:

Stochastic Effects

• No threshold — any dose may (in theory) cause the effect
• Probability increases with dose, but severity is independent of dose
• Linear-no-threshold (LNT) model: The risk of cancer is proportional to the effective dose, even at low doses
• Examples:
• Cancer induction: Lifetime risk ~5% per Sv (ICRP) — i.e., a 10 mSv CT scan increases cancer risk by ~0.05%
• Genetic effects: Mutations in germ cells (risk at low doses is very small)
• Important: CT of the pelvis in a pregnant woman gives a fetal dose ~10–30 mGy; the estimated excess risk of childhood cancer is ~1 in 500–1000 (vs. background ~1 in 500)

5.4 Radiation Protection

Three Principles (ICRP)

1. Justification: The examination must provide a net benefit to the patient
2. Optimisation (ALARA/ALARP): Keep doses As Low As Reasonably Achievable
3. Dose limits: Applied to occupational exposure and members of the public (not to patients — clinical need overrides)

ALARA/ALARP Principles

Time: Reduce exposure time (especially for staff) Distance: Double distance → dose reduced by factor of 4 (inverse square law) Shielding: Lead aprons (0.25–0.5 mm Pb equivalent), lead thyroid shields, lead glasses, mobile lead screens

Staff Protection

• Personal dosimetry: Film badges, thermoluminescent dosimeters (TLDs), optically stimulated luminescence (OSL) dosimeters
• Dose limits (ICRP):
• Occupationally exposed workers: 20 mSv/year (averaged over 5 years), max 50 mSv in any single year
• Pregnant staff: 1 mSv to the fetus over the declared term of pregnancy (once declared, dose limit 1 mSv)
• Public: 1 mSv/year (excluding medical exposures)
• Investigation level: HSE investigation triggered if a pregnant worker receives >1 mSv

IR(ME)R Regulations (Ionising Radiation [Medical Exposure] Regulations)

• Apply to patients (not staff)
• Key requirements:
• All medical exposures must be justified by a practitioner (doctor/dentist)
• Exposures must be optimised (ALARA)
• Referrer (clinician requesting the examination) must provide sufficient clinical information
• Operator must be adequately trained
• Quality assurance programmes must be in place
• Accidental or unintended exposures must be investigated and reported
• Dose constraints: Not defined in IR(ME)R but in national guidance

Lead Shielding

• Lead apron: 0.25 mm Pb equivalent reduces scatter dose by ~90%; 0.5 mm Pb reduces by ~97%
• Thyroid shield: Essential for fluoroscopy operators
• Gonadal shield: Used for patients of reproductive age where gonads are in the primary beam
• Abdominal shield: Used in pregnancy when X-ray is clinically indicated (but collimation is more effective)
• Lead glasses: 0.75 mm Pb equivalent for interventional radiology

Pregnant Staff — Declared Pregnancy

• Once pregnancy is declared in writing to the employer:
• The employer must ensure fetal dose does not exceed 1 mSv for the remainder of the pregnancy
• This usually means the woman cannot work in areas exceeding ~1 mSv/year (i.e., most radiology, interventional, nuclear medicine areas)
• If the woman does not declare, the legal limit is the same as for any worker (20 mSv/year)
• Roles during pregnancy:
• Should not be involved in fluoroscopy/ interventional radiology
• May continue working in areas with background-level exposure (e.g., general wards, ultrasound)
• Pregnant staff in nuclear medicine should avoid handling unsealed sources

5.5 Radiation in Pregnancy

Fetal Radiation Sensitivity by Gestational Age

Key teaching point: - The fetus is most radiosensitive during organogenesis (2–8 weeks) and the early fetal period (8–15 weeks — CNS) - The threshold for deterministic effects (malformations, intellectual disability) is ~100 mGy - Most diagnostic X-ray examinations give a fetal dose well below this threshold

Fetal Doses from Common Examinations

Management of Radiation Exposure in Pregnancy

1. Identify pregnancy: Ask all women of reproductive age if they could be pregnant (NICE, IR(ME)R)
2. Justify the examination: Is it necessary? Can it be delayed until after delivery? Can an alternative (US/MRI) be used?
3. If the examination is necessary (ionising radiation):
4. Use standard technique — do not reduce image quality (risk of missed diagnosis outweighs small benefit of reduced dose)
5. Collimate to the minimum area necessary
6. Use low-dose protocols where available (CT)
7. Avoid multiple phases (single-phase CT rather than triple-phase)
8. Shield the abdomen (but collimation is more important)
9. If fetal dose >100 mGy: Consider the risks with the patient
10. Document the discussion in the medical records

CT Pulmonary Angiogram (CTPA) vs V/Q Scan in Pregnancy

For suspected pulmonary embolism in pregnancy:

RCOG/NICE guidance: Both are acceptable; choice depends on local availability, chest X-ray findings, and patient factors. If chest X-ray is abnormal, CTPA is preferred.

5.6 MRI in Pregnancy

Safety Profile

• No ionising radiation — major advantage
• Thermal effects: RF heating — SAR must be kept within limits (<2 W/kg normal mode)
• Acoustic noise: Gradient switching produces loud noise (up to 130 dB) — hearing protection recommended (fetal hearing development not a contraindication)
• Magnetic field: No proven adverse fetal effects at 1.5 T or 3 T
• AHA/ACOG guidelines: MRI may be used in pregnancy if the information cannot be obtained by ultrasound (or is needed for clinical management)
• First trimester: Theoretical risk of teratogenicity from tissue heating (never proven in humans). Most guidelines suggest avoiding MRI in the first trimester where possible, but it is not contraindicated if clinically necessary

Gadolinium Contrast in Pregnancy

• Crosses the placenta → excreted into amniotic fluid → fetal GI tract absorption
• FDA pregnancy category C: Risk cannot be ruled out
• Animal studies: High-dose gadolinium is teratogenic (not confirmed in humans)
• Neonatal exposure: Gadolinium chelates dissociate slowly in the fetus (immature renal clearance) → potential tissue deposition
• NSF (nephrogenic systemic fibrosis): Risk in mother if she has renal impairment (GFR <30), but not in fetus
• Guidance: Avoid gadolinium in pregnancy unless the information is essential and cannot be obtained by non-contrast MRI or other modalities
• Exceptions: Placenta accreta assessment (some experts use gadolinium for better assessment of myometrial invasion)

Ultrasound vs MRI vs CT in Pregnancy — Summary

6. Hysteroscopy & Imaging in Gynaecology

6.1 Saline Infusion Sonography (SIS) / Sonohysterography

Also called saline infusion sonohysterography (SIS) or simply sonohysterography.

Technique

1. Transvaginal ultrasound probe positioned in the posterior fornix
2. A small catheter (5–7 Fr) is passed through the cervix into the uterine cavity
3. Under ultrasound guidance, sterile normal saline is infused (10–30 mL)
4. The saline distends the uterine cavity, allowing visualisation of the endometrial cavity

Indications

• Abnormal uterine bleeding (AUB) — especially intermenstrual, postmenopausal, menorrhagia
• Subfertility assessment
• Suspected endometrial polyps, submucosal fibroids, intrauterine adhesions (Asherman syndrome)
• Evaluation of endometrial thickness with a clearer view of cavity
• Congenital uterine anomalies (partial assessment)

Findings

• Endometrial polyp: Smooth, echogenic, homogeneous mass within the cavity; vascular stalk on Doppler
• Submucosal fibroid: Hypoechoic mass with acoustic shadowing; overlying endometrium is thin/distorted
• Intrauterine adhesions: Echogenic bands traversing the cavity; partial/complete obliteration of cavity
• Endometrial hyperplasia/carcinoma: Irregular, thickened, heterogeneous endometrium with abnormal vascularity
• Normal: Smooth, regular endometrial cavity

Advantages vs. Hysteroscopy

• Less invasive, no dilation required, cheaper, faster, no anaesthesia
• Can be performed in the outpatient clinic
• Provides information about myometrium and ovaries concurrently
• Better for assessing intramural extent of fibroids

Disadvantages

• Cannot obtain biopsies
• Lower sensitivity for small lesions (<2 mm)
• Cannot treat lesions (resect polyps, fibroids)
• Operator-dependent

6.2 Hysterosalpingography (HSG)

Procedure

• X-ray fluoroscopic examination of the uterine cavity and fallopian tubes
• Performed in the proliferative phase (days 7–10 of menstrual cycle) — after menstruation has stopped, before ovulation (to avoid potential early pregnancy)
• A cannula is placed into the cervix (or a balloon catheter)
• Water-soluble iodinated contrast (e.g., Omnipaque, Isovist) is injected slowly under fluoroscopic guidance
• Sequential images taken as the contrast fills the uterine cavity, then the fallopian tubes, and spills into the peritoneal cavity

Contraindications

• Pregnancy
• Active pelvic inflammatory disease (PID)
• Active vaginal or cervical infection
• Unexplained vaginal bleeding
• Severe iodine allergy (relative: may premedicate)
• Immediate hypersensitivity to contrast (urticaria, anaphylaxis)

Normal Findings

• Uterine cavity: Smooth, regular, inverted triangle shape; filled uniformly
• Fallopian tubes: Fine, smooth, gradually tapering; free intraperitoneal spill of contrast
• Peritoneal spill: Contrast disperses freely around bowel loops and pelvic organs

Abnormal Findings

• Tubal obstruction: Proximal (cornual) or distal (fimbrial); no free spill
• Hydrosalpinx: Dilated, distended tube with no peritoneal spill; "bagel-shaped" or "sausage-shaped" contrast collection
• Intrauterine adhesions: Irregular filling defects
• Endometrial polyp/fibroid: Smooth filling defect in the cavity
• Uterine anomaly: Congenital (arcuate, bicornuate, septate, didelphys, unicornuate)

Limitations

• Cannot assess peritubal adhesions (for this, laparoscopy and dye is superior)
• Does not provide information about the ovaries
• Pain during procedure (cramping)
• Risk of infection (1–3%) — prophylactic antibiotics in high-risk patients (history of PID, dilated tubes)
• Radiation exposure (effective dose ~1–5 mGy)

Therapeutic Effect

• Up to 30% of women conceive within 6 months of HSG (possibly due to "flushing" of debris or mucus plugs)

6.3 Hysterosalpingo-Contrast Sonography (HyCoSy)

Principle

Combines transvaginal ultrasound with contrast medium (positive or negative) to assess tubal patency.

Technique

• Same catheter as SIS
• Contrast used:
• Echovist (galactose-based): A positive contrast agent that appears bright on ultrasound
• Saline + air (microbubbles): Agitated saline creates small air bubbles that reflect ultrasound strongly
• Contrast is injected, and the flow is observed from the uterine cornu through the fallopian tube to the peritoneal cavity

Assessment

• Patent tube: Contrast is seen flowing through the tube and spilling into the peritoneal cavity (contrast fills the pouch of Douglas)
• Blocked tube: Contrast does not pass beyond the cornu or is seen in the tube without spill
• Tubal spasm: Can mimic proximal blockage (significant limitation — up to 10% false-positive rate)
• Hydrosalpinx: Dilated tube with no spill

Advantages vs. HSG

• No ionising radiation
• Can be combined with full pelvic ultrasound (ovaries, endometrium, fibroids)
• Less painful? (similar rates of cramping)
• Outpatient procedure

Disadvantages

• Operator-dependent
• Higher false-positive rate for proximal tubal obstruction (cannot distinguish obstruction from spasm)
• Cannot provide a permanent image record (video recording used)
• Less standardised than HSG

6.4 Hysteroscopy

Principles

Direct endoscopic visualisation of the uterine cavity. Can be diagnostic (to investigate) or operative (to treat).

Equipment

• Hysteroscope: Rigid or flexible; outer diameter 3–10 mm
• Light source: Xenon or LED
• Camera: High-definition CCD/CMOS
• Distension media: See below
• Sheath: Outer sheath allows inflow/outflow of distension media

Distension Media

TURP Syndrome (Transurethral Resection of Prostate Syndrome)

Pathophysiology: Excessive absorption of hypotonic, non-electrolyte distension media (glycine, sorbitol, mannitol) through opened venous channels in the uterine cavity → dilutional hyponatraemia + hypervolaemia + cerebral oedema

Risk factors: - Prolonged operative hysteroscopy (>60–90 minutes) - Large fibroid resection - High intrauterine pressure (>100 mmHg, ideally <70–80 mmHg for saline; <80 mmHg for glycine) - Deep myometrial dissection

Clinical features:

Biochemical features: - Hyponatraemia (Na⁺ < 130 mmol/L → CNS symptoms; <120 → seizures) - Low serum osmolality (<260 mOsm/L) - Hyperglycaemia (glycine → NH₃ conversion) - Metabolic acidosis

Prevention: - Keep intrauterine pressure <75 mmHg (or < mean arterial pressure) - Limit procedure time (<60–90 minutes) - Monitor fluid deficit closely (deficit >1000–1500 mL → stop procedure; deficit >2000 mL → consider termination) - Use automated fluid management systems - Use normal saline with bipolar electrosurgery (reduces risk of dilutional hyponatraemia)

Management: 1. Stop the procedure 2. Administer IV fluids cautiously (may need diuretics if fluid overload) 3. Correct hyponatraemia slowly (<8 mmol/L over 24 hours) — rapid correction risks central pontine myelinolysis 4. Consider IV furosemide (20–40 mg) 5. In severe cases (seizures, coma): IV hypertonic saline (3% NaCl) 6. Monitor serum Na⁺ closely

Complications of Hysteroscopy

Diagnostic vs. Operative Hysteroscopy

Hysteroscopy Findings

6.5 Laparoscopy

Principles

Minimally invasive surgical technique using an endoscope (laparoscope) inserted through the abdominal wall.

Equipment

• Laparoscope: Rigid telescope, 0° or 30° angle, diameter 5–10 mm
• Light source + camera
• Insufflator: Delivers CO₂ gas to create pneumoperitoneum
• Trocars and cannulas: Passage for instruments

Pneumoperitoneum

CO₂ insufflation: - Gas: CO₂ is the preferred insufflant (non-combustible, rapidly absorbed) - Intra-abdominal pressure: Maintained at 12–15 mmHg (higher pressures → haemodynamic compromise) - Flow rate: Initial insufflation at low flow (1–2 L/min) → then high flow (up to 20–40 L/min) - Trendelenburg: Head-down tilt (20–30°) for pelvic access — helps displace bowel from pelvis

Entry Techniques

Veress Needle (Closed Entry)

• Technique: The Veress needle (spring-loaded blunt inner stylet with sharp outer needle) is inserted through a small incision at the umbilicus (or Palmer's point — left upper quadrant).
• Characteristics: Two "clicks" heard as the needle passes through the fascia and peritoneum.
• Confirmation:
• Hanging drop test: A drop of saline placed on the hub is sucked in when the needle tip is in the peritoneal cavity (negative intra-abdominal pressure).
• Aspiration: Check for blood, bowel contents, or urine.
• Gas insufflation: Low initial pressure (<8–10 mmHg) confirms correct placement.
• Complications:
• Bowel injury (~0.1%): Most common serious complication of Veress entry
• Vascular injury (~0.05%): Aortic/iliac vessel puncture
• Preperitoneal insufflation: Gas in the preperitoneal space → emphysema, poor view
• Advantages: Fast, single-step entry
• Disadvantages: "Blind" entry — risk of injury to underlying structures

Hasson Technique (Open Entry)

• Technique: A mini-laparotomy is performed — a 1–2 cm incision is made through the skin and fascia at the umbilicus, the peritoneal cavity is entered under direct vision, and a blunt trocar (Hasson cannula) is inserted.
• Advantages: Safer than Veress (direct vision entry, avoids blind puncture)
• Disadvantages: Slower, larger incision, more CO₂ leak around the cannula, possible difficulty in obese patients
• Preferred: In patients with previous abdominal surgery (risk of adhesions), known pelvic adhesions, or where safety is paramount

Direct Trocar Entry

• Technique: The primary trocar (without prior pneumoperitoneum) is inserted directly through the umbilicus.
• Advantages: Fast, single step
• Disadvantages: "Blind" entry with sharp trocar — risk of vascular/bowel injury
• Not widely recommended — no clear evidence of superiority over Veress

Entry Complications

Complications of Laparoscopy in Gynaecology

Laparoscopy vs. Hysteroscopy — Key Differences

6.6 Colposcopy

Principle

Magnified visualisation of the cervix, vagina, and vulva using a colposcope (a stereoscopic, illuminated microscope with 6–40× magnification).

Indications

• Abnormal cervical screening result (dyskaryosis)
• Clinical suspicion of cervical cancer
• Follow-up after treatment of CIN
• Assessment of DES-exposed women
• Assessment of abnormal-appearing cervix (post-coital bleeding, contact bleeding)

Colposcopic Technique

1. Inspection: View the cervix at low power (6–10× magnification)
2. Saline: Not always used, but helps visualise vessels
3. Acetic acid (3–5%): Applied to the cervix with a cotton swab → denatures cytoplasmic proteins → acetowhite epithelium in areas of CIN/HPV changes (abnormal cells have higher nuclear density → more protein → more acetowhite change)
4. Wait 60–90 seconds for peak effect
5. Acetowhite is transient (fades after ~2 minutes)
6. Lugol's iodine (Schiller's test): Iodine stains glycogen-containing normal squamous epithelium mahogany brown; abnormal epithelium (CIN/HPV) lacks glycogen → iodine-negative (yellow/pale)
7. Biopsy: Punch biopsy from the most abnormal area (using Tischler biopsy forceps)

The Transformation Zone (TZ)

The transformation zone is the region where the original columnar epithelium has been replaced by squamous epithelium through squamous metaplasia. This is the most vulnerable area for HPV-related CIN and cervical cancer.

Types of Transformation Zone (according to IFCPC)

Clinical significance: - Type 1 TZ: Can be treated with excision or ablation - Type 2 TZ: Usually can be treated with excision (may need deeper excision) or ablation if fully visualised - Type 3 TZ: Must be treated with excision (cannot adequately ablate); may require conisation

Colposcopic Grading (Reid Score / Modified Reid Classification)

Total score: - 0–2: CIN 1 (low-grade) - 3–5: CIN 2 (high-grade) - 6–8: CIN 3 (high-grade)

Colposcopic Findings

7. Biophysical Monitoring

7.1 Amniotic Fluid Assessment

Amniotic Fluid Index (AFI)

Technique: - Uterus divided into four quadrants (linea nigra for vertical, umbilicus for horizontal) - The deepest vertical pocket of clear fluid (no cord, no fetal parts) in each quadrant measured - Measurements summed = AFI

Reference ranges:

Classification: - Oligohydramnios: AFI <5 cm (or <5th centile for GA) - Polyhydramnios: AFI >24 cm (or >95th centile for GA) - Anhydramnios: No measurable pocket

Deepest Vertical Pocket (DVP)

• Also called maximum vertical pocket (MVP)
• The single deepest vertical pocket of amniotic fluid (width ≥1 cm)
• Normal: 2–8 cm
• Oligohydramnios: DVP <2 cm
• Polyhydramnios: DVP >8 cm

AFI vs DVP in clinical practice: - AFI has higher sensitivity for oligohydramnios but lower specificity (more false positives) - DVP is simpler, more reproducible, and the preferred method in many guidelines (including NICE for surveillance of post-dates pregnancy) - RCT evidence: Using AFI (vs DVP) does not improve outcomes but increases induction rates

Clinical Significance

Oligohydramnios (DVP <2 cm / AFI <5 cm): - Causes: - Preterm (<37 weeks): PROM (most common), FGR, placental insufficiency, fetal renal anomalies (Potter syndrome — bilateral renal agenesis, polycystic kidney disease, obstructive uropathy), congenital infections, drugs (ACE inhibitors, NSAIDs) - Term (≥37 weeks): Post-dates pregnancy, placental insufficiency - Iatrogenic: Amnioreduction, amniodrainage - Complications: - Early (second trimester): Pulmonary hypoplasia, limb contractures (Potter sequence) - Late (third trimester): Cord compression, meconium aspiration, variable decelerations in labour, increased caesarean section rate - Management: - Preterm + oligohydramnios: Admit for monitoring, corticosteroids for lung maturity, assess for PROM/ infection, serial AF assessment, Doppler (UA, MCA, DV), deliver if signs of fetal compromise - Term + oligohydramnios: Consider induction of labour (if ≥37 weeks). Intrapartum fetal monitoring

Polyhydramnios (DVP >8 cm / AFI >24 cm): - Causes: - Idiopathic (~60%) - Maternal: Diabetes (uncontrolled) - Fetal: GI obstruction (oesophageal/duodenal atresia), tracheoesophageal fistula - Neuromuscular: Myotonic dystrophy, anencephaly (impaired swallowing) - Skeletal dysplasias (restricted chest → impaired swallowing) - Cardiac arrhythmias (fetal tachycardia → hydrops) - Twin-twin transfusion syndrome (recipient twin) - Fetal infection (parvovirus B19, CMV) - Complications: - Preterm labour (overdistension) - Malpresentation (breech, transverse lie) - Cord prolapse (at membrane rupture) - Placental abruption (at rapid decompression) - Post-partum haemorrhage (uterine atony) - Management: - Identify and treat underlying cause - Serial scans for fetal growth, anatomy, amniotic fluid volume - Consider amnioreduction if severe polyhydramnios causing maternal respiratory distress or preterm labour - Third-trimester: increased surveillance for preterm labour, cord prolapse

7.2 Biophysical Profile (BPP)

Components

The BPP comprises five components (modified Manning criteria):

Total score: 10/10

Scoring and Interpretation

Important considerations: - The BPP has a low false-negative rate (fetal death within 1 week of a normal BPP [8–10] is <1/1000) - False-positive rate is higher: up to 50% of fetuses with a score of 6 will be normal - The CTG component (NST) accounts for most false-positive results (fetal sleep cycle → non-reactive CTG) - The modified BPP uses only AFV + CTG (two components, each scored 0–2; total 4/4)

Modified Biophysical Profile

• Components: Only AFV (deepest vertical pocket) + CTG (non-stress test)
• Scoring: Each component scored as normal (2) or abnormal (0)
• Total score: 4/4
• Advantages: Faster than full BPP, less operator-dependent, good screening test
• Disadvantages: Less sensitive than full BPP, no information about fetal tone/movements/breathing

7.3 Kick Charts / Maternal Fetal Movement Counting

Rationale

• Maternal perception of fetal movements is a simple, non-invasive, and cost-effective method to assess fetal well-being
• Reduced fetal movements (RFM) are associated with increased risk of FGR, stillbirth, and adverse outcomes
• RFM: Present in ~10% of pregnancies in the third trimester
• The association between RFM and stillbirth is well-established — an estimated 10–15% of stillbirths may be preceded by RFM

Methods

RCOG Green-Top Guideline (No. 57 — Reduced Fetal Movements)

Key recommendations: - Women should be routinely advised of the importance of fetal movements at the 24-28 week antenatal contact, and again in the third trimester - During the third trimester, women should be aware of their baby's individual pattern of movements - Any woman presenting with RFM should be assessed urgently (same day) - Assessment includes: 1. Symphysial-fundal height (SFH) measurement 2. CTG (≥28 weeks) 3. Ultrasound: fetal biometry + amniotic fluid volume + UA Doppler - If all are normal and movements return to normal → reassure, continue regular follow-up - If any parameter is abnormal → manage accordingly - Recurrent RFM (even with normal investigations) may be associated with increased stillbirth risk → consider induction at 38–39 weeks

Fetal Movement Awareness — Clinical Significance

• Decreased fetal movements may be the earliest presenting sign of placental insufficiency
• Acute RFM: May indicate acute fetal compromise (e.g., abruption, cord accident)
• Chronic RFM: More characteristic of chronic placental insufficiency / FGR
• Increased fetal movements: Not a sign of fetal distress (no association with adverse outcomes)
• Hiccups: Normal, not counted as movements
• Gestational age: Fetal movements peak at ~32 weeks, then plateau/decrease slightly as the uterus becomes more crowded

7.4 Biophysical Monitoring — Summary Table

8. Key Examination Tips & Mnemonics

Essential Equations

Mnemonics

CTG Features — "DR C BRAVADO"

• D — Define risk
• R — Read the CTG from the paper
• C — Contractions
• B — Baseline (110–160 bpm)
• R — Rate (variability)
• A — Accelerations
• V — Variability (5–25 bpm)
• A — Accelerations
• D — Decelerations
• O — Overall assessment (classification)

Causes of Reduced Variability — "SMASHED"

• S — Sleep (fetal sleep cycle ~20–40 min)
• M — Medication (opiates, benzodiazepines, MgSO₄)
• A — Anaesthesia (general, epidural)
• S — Severe hypoxia/acidosis
• H — Heart (fetal heart block)
• E — Extreme prematurity (<28 weeks)
• D — Drugs (maternal CNS depressants)

Causes of Fetal Tachycardia — "CHIMPS"

• C — Chorioamnionitis
• H — Hypoxia (late sign)
• I — Incompetent maternal heart (tachyarrhythmia)
• M — Maternal fever
• P — Prematurity / PE (pulmonary embolism)
• S — Sympathomimetics (terbutaline, salbutamol)

Biophysical Profile Components — "FMF ACT"

• F — Fetal breathing movements
• M — Fetal movements
• F — Fetal tone
• A — Amniotic fluid volume
• C — Cardiotocography

Oligohydramnios Causes — "PRROM"

• P — Placental insufficiency (FGR)
• R — Renal anomalies (Potter)
• R — Rupture of membranes (PROM)
• O — Obstructive uropathy
• M — Medications (ACE inhibitors, NSAIDs, indomethacin)

Polyhydramnios Causes — "FID GAS"

• F — Fetal GI obstruction / malformations
• I — Idiopathic
• D — Diabetes (maternal)
• G — Genetic/dysmorphic (aneuploidy)
• A — Anaemia / hydrops
• S — Skeletal dysplasias / structural anomalies (CNS, thoracic)

Normal Fetal Heart Rate — "110–160"

The normal baseline is 110–160 bpm. The phrase "Everything in Between is Great" = EB (everything between) = 110–160 E in B pm? More simply: "One Ten to One Sixty — That's Properly Healthy" (110–160 = PH = Pulse Healthy).

Deterministic vs Stochastic — "DS"

• D — Deterministic: Dose threshold, Severity increases, Deterministic
• S — Stochastic: No threshold, Severity independent, Probability increases

Three Principles of Radiation Protection — "JOT"

• J — Justification
• O — Optimisation (ALARA)
• T — Time, Distance, Shielding

TURP Syndrome Features — "HAS BAD"

• H — Hyponatraemia
• A — Altered mental state
• S — Seizures
• B — Bradycardia
• A — Airway compromise
• D — Diuresis (furosemide)

Quick-Fire Key Facts (MRCOG Part 1)

1. Ultrasound speed in soft tissue: 1540 m/s
2. Piezoelectric effect: Conversion of electrical → mechanical energy (and vice versa)
3. ALARA: As Low As Reasonably Achievable
4. Pulse-echo principle: Distance = (c × time) / 2
5. B-mode: Brightness mode (2D greyscale)
6. Axial resolution determined by: Spatial pulse length (higher frequency = better)
7. Lateral resolution determined by: Beam width at focal zone
8. Acoustic shadowing occurs behind: High-impedance interfaces (bone, stone)
9. Acoustic enhancement occurs behind: Fluid-filled structures (cyst, bladder)
10. Crown-rump length (CRL): Most accurate dating method; 7–13+6 weeks
11. CRL ≥ 7 mm + no cardiac activity = miscarried
12. MSD ≥ 25 mm + no fetal pole = miscarried
13. NT measured at: 11+0 to 13+6 weeks (CRL 45–84 mm)
14. NT > 99th centile: Risk of trisomy 21, 18, 13, cardiac anomalies, Noonan syndrome
15. Lambda sign: Dichorionic twins
16. T-sign: Monochorionic twins
17. MCA PSV > 1.5 MoM: Fetal anaemia
18. Umbilical artery AEDF/REDF: Severe placental insufficiency
19. DV reversed a-wave: Impending fetal compromise
20. CPR = MCA PI / UA PI; <1.0 = abnormal
21. CTG normal baseline: 110–160 bpm
22. Normal variability: 5–25 bpm
23. Late decelerations: Uteroplacental insufficiency
24. Variable decelerations: Cord compression
25. Sinusoidal pattern: Fetal anaemia (or narcotics)
26. FBS pH > 7.25 = normal; < 7.20 = acidosis → deliver
27. ALARA/ALARP: Three principles: justification, optimisation, dose limits
28. Deterministic effects: Threshold; severity increases with dose
29. Stochastic effects: No threshold; cancer risk
30. Fetal safe threshold for radiation: <100 mGy
31. Pregnant staff dose limit: 1 mSv to fetus
32. Hypospadias correction: <3° chordee, proximal meatus at coronal sulcus — surgery at 6–18 months
33. TURP syndrome: Hyponatraemia from absorption of hypotonic distension media
34. CO₂ insufflation pressure: 12–15 mmHg for laparoscopy
35. Saline infusion sonography (SIS): For endometrial cavity assessment
36. Colposcopy: Acetic acid → acetowhite; Lugol's iodine → Schiller's test
37. Transformation zone: Squamocolumnar junction, vulnerable to HPV
38. Reid score: Margin + colour + vessels + iodine for CIN grading
39. Amniotic fluid assessment: DVP (<2 cm = oligo, >8 cm = poly) or AFI (<5 cm = oligo, >24 cm = poly)
40. BPP: 5 components; 8–10 = normal; 0–2 = immediate delivery
41. Reduced fetal movements (RFM): 10% of pregnancies; associated with FGR/stillbirth
42. First-trimester Combined Screening: NT + β-hCG + PAPP-A at 11–13+6 weeks
43. Hadlock formula: Most widely used for EFW (AC, FL, HC, BPD)
44. ISUOG guidelines: UA Doppler for FGR (Grade A); MCA Doppler (Grade B); not for low-risk screening
45. Umbilical artery Doppler progression in FGR: PI↑ → AEDF → REDF → DV a-wave reversal → fetal demise

References & Further Reading

1. RCOG Green-Top Guidelines:
2. No. 31: The Investigation and Management of the Small-for-Gestational-Age Fetus
3. No. 57: Reduced Fetal Movements
4. No. 7: Antenatal Corticosteroids to Reduce Neonatal Morbidity and Mortality

5. No. 19: Umbilical Cord Prolapse

6. NICE Guidelines:

7. CG62: Antenatal care
8. NG133: Hypertension in pregnancy
9. NG201: Intrapartum care

10. QS22: Quality standard for antenatal care

11. FIGO 2015: Intrapartum Fetal Monitoring Guidelines

12. ISUOG Practice Guidelines:

13. Use of Doppler ultrasonography in obstetrics
14. Performance of the routine mid-trimester fetal ultrasound scan

15. Ultrasound assessment of fetal biometry and growth

16. BMUS Guidelines: Safety of ultrasound (thermal index, mechanical index, scanning times)

17. ICRP Publication 103: The 2007 Recommendations of the International Commission on Radiological Protection

18. IR(ME)R 2017: Ionising Radiation (Medical Exposure) Regulations

19. NICE Quality Standard QS68: Fertility problems — includes HSG recommendations

20. Mari G, Deter RL, Carpenter RL, et al. Noninvasive diagnosis by Doppler ultrasonography of fetal anemia due to maternal red-cell alloimmunization. Collaborative Group for Doppler Assessment of the Blood Velocity in Anemic Fetuses. N Engl J Med. 2000;342(1):9-14.

21. Hadlock FP, Harrist RB, Sharman RS, et al. Estimation of fetal weight with the use of head, body, and femur measurements—a prospective study. Am J Obstet Gynecol. 1985;151(3):333-337.

22. Quintero RA, Morales WJ, Allen MH, et al. Staging of twin-twin transfusion syndrome. J Perinatol. 1999;19(8 Pt 1):550-555.

23. TRUFFLE Study Group. A randomized trial of timed delivery for the compromised preterm fetus: short-term outcomes. Ultrasound Obstet Gynecol. 2015;45(6):651-660.

24. Manning FA, Morrison I, Lange IR, et al. Fetal assessment based on fetal biophysical profile scoring: experience in 12,620 referred high-risk pregnancies. Am J Obstet Gynecol. 1985;151(3):343-350.

25. NICE Guideline NG201: Fetal monitoring in labour (2022 update — 3-tier classification)

Disclaimer: This document is intended as a study aid for MRCOG Part 1 preparation. It summarises key concepts from multiple authoritative sources. Candidates should consult the latest RCOG, NICE, ISUOG, and FIGO guidelines for current practice recommendations.