Index

MRCOG Part 1: Reproductive Physiology — Comprehensive Deep-Dive Study Guide

Target: 20,000+ words of exam-focused, integrated physiology Scope: Menstrual cycle → HPO axis → Ovulation → Fertilisation & Implantation → Maternal adaptations → Lactation → Puberty → Menopause Format: Tables, diagrams (text), clinical correlations, mnemonics, exam pearls

Table of Contents

1. The Menstrual Cycle
2. Hypothalamic-Pituitary-Ovarian (HPO) Axis
3. Ovulation
4. Fertilisation & Implantation
5. Maternal Physiological Adaptations to Pregnancy
6. Lactation
7. Puberty
8. Menopause & Climacteric
9. Appendix: Exam Mnemonics & Key Numbers

1. The Menstrual Cycle

1.0 Foundational Concepts in Reproductive Endocrinology

Before diving into cycle mechanics, it is essential to understand the steroid hormone synthesis cascade, receptor dynamics, and principles of feedback regulation that underpin the entire HPO axis.

Steroid Hormone Chemistry: - All gonadal steroids derive from cholesterol (27-carbon precursor) - Cholesterol is transported into the inner mitochondrial membrane by StAR protein (Steroidogenic Acute Regulatory protein) — this is the rate-limiting step in steroidogenesis - Conversion: cholesterol (C27) → pregnenolone (C21) → progesterone (C21) → androstenedione (C19) → oestradiol (C18) - The aromatase enzyme (CYP19A1) is the only enzyme that can convert androgens to oestrogens — it is expressed in granulosa cells, placenta, adipose tissue, bone, and brain

Oestrogen Receptor (ER) Biology: - Two subtypes: ERα (mostly uterus, breast, hypothalamus) and ERβ (ovary, lung, prostate, colon) - Both are nuclear receptors that act as ligand-activated transcription factors - Upon oestrogen binding, ER dimerises, translocates to nucleus, binds to oestrogen response elements (EREs) on DNA - ERα is critical for positive feedback and uterine growth; ERβ may have modulating/antagonistic roles - Selective oestrogen receptor modulators (SERMs): tamoxifen (ER antagonist in breast, agonist in bone/uterus), raloxifene (antagonist in breast/uterus, agonist in bone)

Progesterone Receptor (PR) Biology: - Two isoforms: PR-A and PR-B (from same gene, different promoters) - PR-A: represses PR-B and ER activity (anti-oestrogenic) - PR-B: full transcriptional activator - Progesterone action requires PR-B; PR-A modulates the response - In endometrium, progesterone opposes oestrogen-driven proliferation → secretory transformation

Metabolism and Clearance: - Oestradiol is converted in the liver to oestrone → oestriol → conjugated with glucuronide/sulphate → renal excretion - Progesterone → pregnanediol (urinary metabolite) — historically measured to confirm ovulation - 2-Hydroxyoestrone (catechol oestrogen): has anti-oestrogenic properties; formed by CYP1A1

1.1 Overview

The menstrual cycle is a ~28-day (range 21–35 days) sequence of coordinated endocrine and structural events that prepares the female reproductive tract for pregnancy. It is divided into:

• Ovarian cycle (follicular phase, ovulation, luteal phase)
• Endometrial cycle (menstrual, proliferative, secretory, ischaemic)
• Cervical cycle (mucus changes)
• Vaginal cycle (cytological changes)
• Hormonal cycle (GnRH, FSH, LH, oestrogen, progesterone, inhibins, activin, follistatin)

1.2 The Ovarian Cycle

1.2.0 Ovarian Follicle Reserve and AMH

Anti-Müllerian Hormone (AMH) — The Ovarian Reserve Marker:

The PI3K/Akt/mTOR Signalling in Follicle Activation: - Primordial follicles are kept quiescent by FOXO3a (transcription factor) - PI3K activation → phosphorylates FOXO3a → FOXO3a excluded from nucleus → follicle activation - PTEN (tumour suppressor) dephosphorylates PIP3 → antagonises PI3K → maintains quiescence - Disruption: Pten knockout → global follicle activation → premature ovarian failure - mTORC1 integrates growth signals → granulosa cell proliferation - This pathway is targeted for infertility treatments (e.g., in vitro activation — IVA — for POI)

1.2.1 Follicular Phase (Days 1–14, variable)

Folliculogenesis takes ~120 days from primordial follicle recruitment to ovulation. Only the final ~50 days are gonadotrophin-dependent.

Stages of Follicular Development:

Key Concepts:

1. Primordial follicle pool: Established in fetal life (~6–7 million at 20 weeks, ~1–2 million at birth, ~300,000–500,000 at menarche, <1000 at menopause). Non-renewable.

2. Initial recruitment: Continuous, gonadotrophin-independent process where primordial follicles enter the growing pool. Regulated by local factors (Kit ligand/c-Kit, PI3K/Akt/mTOR pathway, AMH inhibits recruitment).

3. Cyclic recruitment: At the start of each cycle, a cohort (5–20) of antral follicles (2–5 mm) is rescued from atresia by rising FSH. This is the "FSH window" — the critical period when FSH concentration governs which follicles survive.

4. Dominant follicle selection: The follicle with the lowest threshold for FSH (most FSH receptors, highest oestradiol production) survives. It produces increasing oestradiol + inhibin A/B, which:

5. Suppress FSH via negative feedback (withdraws support from smaller follicles)
6. Sensitise itself to FSH via upregulation of its own FSH receptors

7. Induce LH receptors on granulosa cells (preparing for luteinisation)

8. Atresia: 99.9% of follicles undergo atresia. Mechanism: granulosa cell apoptosis (Fas/FasL system, caspases). Small follicles → theca cells degenerate. Large antral follicles → granulosa apoptosis triggered by falling FSH.

1.2.2 The 2-Cell 2-Gonadotrophin Theory of Oestrogen Synthesis

This is essential MRCOG knowledge. Understand both the cellular compartmentalisation and the pathway.

                    CHOLESTEROL
                         |
                    Pregnenolone
                         |
   LUTEINISING HORMONE (LH) ---> 17α-OH-Pregnenolone
         (theca cells)                |
                                 Dehydroepiandrosterone (DHEA)
                                         |
                                     Androstenedione   ← DIFFUSES across basement membrane
                                         |
                    FOLLICLE-STIMULATING HORMONE (FSH) ---> Androstenedione → OESTRONE (E1)
                        (granulosa cells)                     ↓               ↓
                                                        TESTOSTERONE → OESTRADIOL (E2)
                                                         (via aromatase CYP19A1)

Clinical Correlation — Polycystic Ovary Syndrome (PCOS): - LH hypersecretion → theca overstimulation → hyperandrogenism - Relative FSH deficiency → impaired aromatisation → androgens accumulate - Result: anovulation, arrested follicular growth

1.2.3 Luteal Phase (Days 15–28, fixed duration ~14 days)

After ovulation, the ruptured follicle transforms into the corpus luteum under the influence of LH.

Corpus Luteum Formation: 1. Follicle wall collapses 2. Granulosa cells → large luteal cells (progesterone-producing, >20 μm, LH-responsive) 3. Theca interna cells → small luteal cells (androgen-producing, <20 μm, LH-responsive) 4. Angiogenesis — new capillaries invade from theca (VEGF-driven) 5. Accumulation of yellow lutein pigment (carotenoids) → gross yellow appearance

Corpus Luteum Function:

Corpus Luteum Rescue vs Luteolysis:

Luteolysis mechanism: - PGF2α (uterine origin, possibly via lysophosphatidic acid) - ↓ LH receptors, ↓ cAMP - ↑ Free radicals (reactive oxygen species) - ↑ Apoptosis (caspase 3/9 activation) - Structural involution → corpus albicans (fibrous scar)

1.3 The Endometrial Cycle

1.3.1 Endometrial Layers

                    ENDOMETRIUM (3 mm pre-menstrual → 12 mm mid-secretory)
                    -----
     ___________________|___________________
    |                      |                |
  STRATUM              STRATUM          STRATUM
  BASALIS              FUNCTIONALIS     SPONGIOSUM
  (not shed)           (shed with       (shed with
                        menstruation)    menstruation)

1.3.2 Phases of the Endometrial Cycle

Detailed Histology for MRCOG:

1.4 Coordinated Hormonal Changes

1.4.1 Endocrine Profile Across the Cycle

1.4.2 Inhibin-Activin-Follistatin System

This is a fine-tuning system for FSH regulation, often examined.

Key Points: - Inhibin B peaks in early follicular phase and reflects the cohort of recruited follicles → FSH falls - Inhibin A rises in luteal phase from corpus luteum → suppresses FSH during luteal phase - Activin stimulates FSHβ-subunit gene expression in pituitary - Follistatin prevents activin from binding its receptor → terminates FSH stimulation - Ratio of activin:follistatin determines FSH output

1.5 Mechanisms of Menstruation

1.5.1 Prostaglandin Synthesis Pathway in Endometrium

  MEMBRANE PHOSPHOLIPIDS
       ↓ (Phospholipase A2 — activated by steroid withdrawal, cytokines)
  ARACHIDONIC ACID
       ↓
  COX-1 (constitutive)    COX-2 (inducible — ↑ by IL-1, TNF-α)
       ↓                          ↓
  PGG₂ → PGH₂                    PGG₂ → PGH₂
       ↓                                  ↓
  PGF₂α synthase → PGF₂α            PGE synthase → PGE₂
  TX synthase → TxA₂                PGI synthase → PGI₂ (prostacyclin)

  PGF₂α: VASOCONSTRICTOR (spiral artery spasm)
  PGE₂: VASODILATOR + myometrial contraction
  PGI₂: VASODILATOR + anti-platelet aggregation
  TxA₂: Vasoconstrictor + platelet aggregation

The balance between PGF₂α (vasoconstrictor) and PGE₂ (vasodilator) determines menstrual blood loss. In menorrhagia, PGE₂/PGF₂α ratio is increased → less vasospasm → more bleeding.

COX-1 vs COX-2: - COX-1: Constitutive, in endometrium throughout cycle - COX-2: Induced at ovulation and during menstruation; targeted by NSAIDs to reduce menstrual bleeding - NSAIDs (mefenamic acid, naproxen) reduce menstrual blood loss by 20–50% (inhibit both COX-1 and COX-2, but COX-2 inhibition is key)

1.5.2 Matrix Metalloproteinases (MMPs)

The MMP family includes >20 zinc-dependent endopeptidases that degrade all components of the extracellular matrix. In menstruation:

Regulation: - TIMPs (Tissue Inhibitors of Metalloproteinases): TIMP-1, -2, -3, -4 - Progesterone withdrawal → ↓ TIMPs → ↑ net MMP activity - Progesterone in luteal phase maintains TIMPs → tissue stability - This is why progesterone supplementation in early pregnancy stabilises endometrium

Clinical Correlation — Endometriosis: - Endometriotic lesions have ↑ MMP activity and ↓ TIMP expression → increased invasiveness - Retrograde menstruation + impaired clearance of menstrual debris + ↑ MMP activity → lesion formation - Progestins treat endometriosis partly by ↑ TIMPs → ↓ MMP activity → lesion stabilisation

1.5.3 Sequence of Events in Menstruation

Progesterone and oestradiol withdrawal at the end of the luteal phase triggers a cascade:

Falling P4 + E2
       ↓
  ↑ Prostaglandins (PGF2α > PGE2)
  ↑ Endothelin-1
  ↑ Vasoconstrictors
       ↓
  Spiral artery vasospasm (PGF2α-mediated, rhythmic — every 6–12 hours)
       ↓
  Ischaemia + hypoxia
       ↓
  ↑ Matrix Metalloproteinases (MMPs) — especially MMP-1, MMP-3, MMP-9
  ↑ Leucocyte infiltration (neutrophils, macrophages, eosinophils)
  ↑ Cytokines (IL-1, IL-8, TNF-α)
       ↓
  ECM degradation + tissue breakdown
  Release of tissue factor + vasodilators (PGE2, PGI2) → bleeding
       ↓
  Haemostatic plug formation → fibrin deposition
  Vasoconstriction again → bleeding stops temporarily
  Repeat cycle → pieces of functionalis detach and are shed

1.5.4 Key Mediators

1.5.5 Haemostatic Mechanisms in Menstruation

Despite tissue destruction, menstrual blood loss is normally 30–50 mL/cycle (>80 mL = menorrhagia). Control mechanisms:

1. Spiral artery vasoconstriction — limits blood flow
2. Platelet plug formation — activated by exposed ECM
3. Fibrin deposition — intrauterine clots form but are lysed by fibrinolysis (hence menstrual blood is usually non-clotting due to high fibrinolytic activity; clots in menses indicate heavy bleeding)
4. Endometrial repair — re-epithelisation begins within 24–36 hours of onset, driven by oestradiol-stimulated proliferative phase

Clinical Correlation — Menorrhagia: - Disturbed prostaglandin balance (↑ vasodilatory PGE2 relative to PGF2α) - Disorders of haemostasis (von Willebrand disease) - Intrauterine pathology (fibroids, polyps, adenomyosis)

1.6 Cervical Changes

1.6.1 Cervical Mucus

The cervix produces 20–60 mg of mucus daily, increasing to >500 mg/day at ovulation under oestrogen influence.

1.6.2 Ferning Pattern

Dried cervical mucus forms a characteristic fern-like pattern under low-power microscopy. Requires: - High NaCl concentration (oestrogen-induced) - High water content - Absence of progesterone (progesterone disrupts Na⁺ binding to mucin)

Clinical uses: Ovulation detection (ferning peaks at ovulation), assessment of oestrogen status.

1.6.3 Spinnbarkeit

Ability of cervical mucus to be drawn into a thread. Peaks at ovulation (≥10 cm stretch). Reflects the linear alignment of mucin polymers under oestrogen dominance.

1.7 Fallopian Tube Motility and Secretion

Tubal Secretions: - Produced by secretory epithelial cells - Volume: 0.1–2 mL/day - Composition: Na⁺, K⁺, Cl⁻, HCO₃⁻, glucose, lactate, pyruvate, amino acids, proteins (including oviduct-specific glycoprotein — OVGP1) - Function: Provides gamete transport medium, supports early embryo development, capacitation of sperm, nutrition

Sperm Transport: - Rapid phase: within 5–10 minutes after intercourse (uterine + tubal contractions) - Sustained phase: sperm reach ampulla within 30–60 minutes - Sperm reservoir: cervical crypts (can survive 48–72 hours, up to 5 days) - Isthmic-ampullary junction: sperm are released in small numbers near ovulation

Oocyte Transport: - Ovum pick-up by fimbriae (ciliary action + fimbrial contraction) - Transport through ampulla (3–4 days) - Fertilisation occurs in ampullary-isthmic junction (within 12–24 hours of ovulation) - Delayed at isthmus (4–5 days) — time for development to morula/blastocyst

Clinical Correlation — Ectopic Pregnancy: - Tubal damage (infection, surgery) → impaired motility → implantation within tube - Smoking → ↓ ciliary beat frequency → ↑ ectopic risk - Progesterone-only contraception → altered tubal motility

2. Hypothalamic-Pituitary-Ovarian (HPO) Axis

2.1 Anatomy of the HPO Axis

          HIGHER CENTRES
         (cortex, limbic system)
                ↓
        HYPOTHALAMUS
    (Arcuate nucleus, POA)
     GnRH pulse generator
          ↓ (GnRH)
   PORTAL CIRCULATION
          ↓
     ANTERIOR PITUITARY
      (Gonadotropes)
      ↓ (FSH + LH)
       OVARY
  (Follicle, Corpus luteum)
      ↓ (E2, P4, Inh A/B)
    Feedback to hypothalamus + pituitary

2.2 GnRH (Gonadotrophin-Releasing Hormone)

2.2.1 GnRH Neurons — Origin and Migration

GnRH neurons have a unique embryological origin: they originate in the olfactory placode (medial nasal epithelium) and migrate along the olfactory nerve fibres and the terminal nerve (cranial nerve 0) into the forebrain, settling in the preoptic area and arcuate nucleus of the hypothalamus.

Clinical Relevance — Kallmann Syndrome: - Failure of GnRH neuron migration → GnRH deficiency → hypogonadotrophic hypogonadism - KAL1 (anosmin-1): X-linked, cell adhesion molecule required for migration - FGFR1/FGF8: Autosomal, fibroblast growth factor signalling for olfactory/GnRH development - PROK2/PROKR2: Prokineticin signalling for migration - Associated: anosmia/hyposmia, cleft lip/palate, renal agenesis (KAL1), dental agenesis (FGFR1), synkinesia (mirror movements — KAL1), hearing loss - Treatment: Pulsatile GnRH pump or gonadotrophin therapy for fertility

2.2.2 GnRH Pulse Generator — Detailed Neuroanatomy

Located in the arcuate nucleus of the mediobasal hypothalamus. The pulse generator produces episodic electrical discharges every 30–120 minutes, which trigger GnRH release into the portal system.

KNDy Neurons — Essential for Pulse Generation:

       KISSPEPTIN
          |
    Neurokinin B (NKB)
          |
      Dynorphin
          |
    KNDy Neuron (arcuate nucleus)
          |
        GnRH Neuron
          |
      GnRH released

Model of Pulse Generation: 1. NKB stimulates kisspeptin release from KNDy neurons 2. Kisspeptin stimulates GnRH neuron firing → GnRH pulse 3. After the pulse, dynorphin provides negative feedback to terminate → refractory period 4. Progesterone upregulates dynorphin → slows pulse frequency in luteal phase

Clinical Correlations:

2.3 Gonadotropes — FSH and LH

2.3.1 Biosynthesis

Key Concept: FSH, LH, hCG, and TSH share a common α-subunit (92 amino acids). Specificity is conferred by the β-subunit.

FSHβ regulation: Stimulated by activin, inhibited by inhibin and follistatin. LHβ regulation: Stimulated by GnRH pulses.

2.3.2 Effect of GnRH Pulse Frequency

This is a core MRCOG concept — pulse frequency determines which gonadotrophin is secreted.

Molecular mechanism: - High frequency pulses → preferential activation of p38/MAPK → ↑ LHβ transcription - Low frequency pulses → preferential activation of Smad3/activin pathway → ↑ FSHβ transcription

2.3.3 Gonadotrophin Action

2.4 Feedback Loops

2.4.1 Negative Feedback — Molecular Detail

Mechanism of Oestradiol Negative Feedback in the Follicular Phase:

In the early-mid follicular phase, low but rising oestradiol binds to ERα on KNDy neurons in the arcuate nucleus. This activates the opioid system (dynorphin) within KNDy neurons, which then inhibits kisspeptin-NKB signalling via κ-opioid receptors (KOR). The result is a reduction in GnRH pulse frequency, which favours FSH over LH secretion. This is paradoxical at first glance (oestradiol in early follicular suppresses gonadotrophins) and is the classic negative feedback.

Progesterone Negative Feedback — The Luteal Phase Brake:

Progesterone exerts its dominant negative feedback effect in the luteal phase: 1. Progesterone binds to PR in KNDy neurons → ↑ dynorphin expression 2. Dynorphin acts via κ-opioid receptors on KNDy neurons themselves → autoinhibition 3. ↓ Kisspeptin release → ↓ GnRH pulse frequency (slows from ~60 min to ~240 min) 4. Slower pulses favour FSHβ expression → FSH rises slightly in late luteal (prepares next cycle cohort)

Clinical Correlation — Progesterone-Only Contraception: - Continuous progestin (POP, implant, IUS) → sustained ↑ dynorphin → marked ↓ GnRH pulse frequency → ↓ LH → anovulation - Additionally: cervical mucus thickening, endometrial atrophy, altered tubal motility - Progestin-only pills inhibit ovulation in only ~40–60% of cycles (thick mucus is the primary mechanism for some preparations)

Testosterone Negative Feedback in Males: - Testosterone → aromatised to oestradiol → negative feedback on GnRH/LH - Testosterone also → binds directly to androgen receptors → negative feedback on LH (but less effect on FSH) - Inhibin B from Sertoli cells specifically suppresses FSH

2.4.2 Positive Feedback (Oestradiol)

This is the mechanism that generates the LH surge and is a high-yield MRCOG topic.

Requirements for Positive Feedback: 1. Oestradiol > 200 pmol/L (some sources: > 750–1000 pmol/L) for 36–48 hours sustained 2. The dominant follicle must reach maturity (~18–22 mm) 3. Progesterone must remain low (any progesterone rise blocks positive feedback) 4. Adequate pituitary gonadotrope responsiveness

Mechanism:

High, sustained oestradiol
       ↓
Binds to ERα in:
  (a) Hypothalamus (anteroventral periventricular nucleus — AVPV)
      → ↑ Kisspeptin expression → ↑ GnRH neuronal firing
      → Massive GnRH release → LH surge
  (b) Anterior pituitary
      → ↑ LHβ transcription directly
      → ↑ GnRH receptor expression on gonadotropes
      → ↑ Sensitivity to GnRH
       ↓
  LH SURGE (LH peaks ~14–48 hours after oestradiol peak)

Why does the same oestradiol cause negative then positive feedback? - Prolonged exposure to high levels switches ER-mediated signalling - Negative feedback: ERβ maybe predominant in some regions - Positive feedback: ERα in AVPV kisspeptin neurons (these have a high threshold for oestradiol) - Rising levels recruit more ERα → eventually cross threshold → switch

What about progesterone? - Progesterone blocks positive feedback (hence oral contraceptive pills, which contain progestin, prevent ovulation) - Progesterone's effect: ↑ dynorphin → slows GnRH pulses → cannot sustain surge mode

Clinical Correlation — Anovulation: - PCOS: tonic LH high but no surge (oestradiol not sustained high enough; elevated progesterone or androgens interfere) - Weight loss/stress: no GnRH pulses → insufficient follicle growth → low oestradiol → no surge - GnRH antagonists: block positive feedback

3. Ovulation

3.1 Sequence of Events

Ovulation occurs ~38 hours after the onset of the LH surge and ~10–12 hours after the LH peak.

    LH SURGE (Day 13–14, 08:00–10:00)
                ↓
        Resumption of meiosis I
    (germinal vesicle breakdown)
                ↓
        First polar body extrusion
        (Metaphase II arrested)
                ↓
        Luteinisation of granulosa cells
    (↑ progesterone production despite no luteal phase yet)
                ↓
        Prostaglandin cascade (↑ COX-2, ↑ PGE2, ↑ PGF2α)
                ↓
        Proteolytic enzyme activation
    (plasminogen → plasmin; MMPs → collagen breakdown)
                ↓
        Follicular rupture
    (stigma formation → extrusion of oocyte-cumulus complex)

3.2 The LH Surge — Detailed Mechanisms

3.2.1 Oocyte Maturation and Meiosis Regulation

The oocyte is arrested at prophase I from fetal life until the LH surge. This arrest is maintained by high intra-oocyte cAMP levels produced by the Gₛ-coupled GPR3/12 receptor on the oocyte membrane, which is activated by an unidentified factor from granulosa cells. cAMP activates PKA, which prevents activation of maturation-promoting factor (MPF = CDK1 + cyclin B).

The LH Surge Triggers Meiosis Resumption:

  LH surge
       ↓
  LH binds to granulosa cells (not oocyte — oocyte has no LH receptors)
       ↓
  ↓ cGMP production in granulosa cells
  cGMP diffuses into oocyte via gap junctions
       ↓
  ↓ Intra-oocyte cGMP → ↓ PDE3A inhibition
  PDE3A becomes active → breaks down cAMP
       ↓
  ↓ Intra-oocyte cAMP → ↓ PKA activity
       ↓
  Activation of MPF (CDK1 + cyclin B)
  Dephosphorylation of CDK1 (by CDC25 phosphatase)
       ↓
  **Germinal Vesicle Breakdown (GVBD)** — nuclear envelope dissolves
  Chromosomes condense, spindle forms
  Meiosis I proceeds → first polar body extruded
       ↓
  **Meiosis II begins, arrests at metaphase II**
  (Arrest maintained by **CSF** — Cytostatic Factor = Mos/MEK/MAPK pathway)
       ↓
  Fertilisation → Ca²⁺ oscillations → CSF inactivation → Meiosis II completes → second polar body

Key Molecular Players: - GPR3/GPR12: Oocyte membrane receptors that maintain high cAMP through Gₛ - PDE3A: Phosphodiesterase that hydrolyses cAMP; inhibited by cGMP - MPF (Maturation-Promoting Factor): CDK1 + Cyclin B — the universal trigger for M-phase entry - Mos/MEK/MAPK (CSF pathway): Maintains metaphase II arrest until fertilisation - CDC25: Phosphatase that activates CDK1 by removing inhibitory phosphates - WEE1/MYT1: Kinases that inactivate CDK1 by adding inhibitory phosphates

Clinical Correlation — Oocyte Ageing: - With maternal age, the oocyte's ability to maintain metaphase II arrest declines - ↑ Risk of aneuploidy (meiotic spindle instability, premature sister chromatid separation) - This is why trisomy risk ↑ with maternal age (especially >35 years) - Oocytes are more prone to nondisjunction in meiosis I

3.2.2 The Two-Factor Model of Follicular Rupture

1. Intrafollicular pressure does NOT increase significantly — not a burst mechanism
2. Proteolytic digestion of the follicular wall at the stigma + thecal smooth muscle contraction (PGF2α-mediated) → expulsion of oocyte

3.3 Timing of Ovulation

Cycle day in 28-day cycle: - Ovulation: Day 14 (range Day 11–17) - Follicular phase: variable length (10–20 days) - Luteal phase: fixed ~14 days (± 2 days)

Clinical detection: - BBT rise (progesterone is thermogenic) — shift occurs after ovulation (by 1 day) - Urinary LH kits — detect LH surge 24–36 hours before ovulation - Ultrasound — follicular disappearance

3.4 Ovum Pick-Up

Mechanisms: 1. Fimbriae sweep over the ovarian surface at ovulation (guided by fimbrial-ovarian ligament) 2. Fimbrial cilia beat creates a gentle current toward the tubal ostium 3. Peritoneal fluid flow (variable negative intra-abdominal pressure during inspiration) 4. Oocyte-cumulus complex is sticky and adheres to fimbrial surface

Key fact: The fallopian tube does NOT "suck" the oocyte. The fimbriae physically grasp the ovary around the time of ovulation (fimbrial-ovarian apposition) and the oocyte is transported by ciliary action.

3.5 Corpus Luteum Formation (Recap with Details)

3.6 Luteal Phase Length — Why Fixed?

The corpus luteum has a programmed lifespan of ~14 days. This is intrinsic to the granulosa-luteal cells and is independent of the pituitary (removal of the pituitary in luteal phase does not prevent regression). However, hCG can "rescue" it.

Mechanism of fixed lifespan: - LH receptors progressively decline after day 5 - cAMP responsiveness decreases - Local factors (PGF2α, cytokines) accumulate and trigger apoptosis - "Luteolytic cascade" pre-programmed

4. Fertilisation & Implantation

4.1 Fertilisation

4.1.1 Sperm Capacitation

Definition: Final maturation of sperm in the female reproductive tract that enables them to fertilise an oocyte. Takes place in the uterine cavity and fallopian tube, typically over 4–6 hours.

Location: Primarily in the fallopian tube (isthmus-ampulla region). Tubal secretions contain factors that promote capacitation (albumin, HCO₃⁻, Ca²⁺).

4.1.2 Acrosome Reaction

Definition: Exocytosis of the acrosome contents (hydrolytic enzymes) enabling sperm to penetrate the zona pellucida.

Trigger: Binding of sperm to ZP3 glycoprotein on the zona pellucida.

  Sperm binds ZP3 (acrosomal membrane → ZP3 interaction)
       ↓
  Tyrosine kinase signalling cascade activated
       ↓
  ↑ Intracellular Ca²⁺ (T-type Ca²⁺ channels open)
  ↑ Intracellular pH (Na⁺/H⁺ exchange)
       ↓
  Fusion of outer acrosomal membrane with plasma membrane
       ↓
  **Acrosome reaction** — enzyme release:
    • Acrosin (trypsin-like serine protease)
    • Hyaluronidase (digests cumulus ECM)
    • Zona lysin
       ↓
  Sperm penetrates zona pellucida via:
    (1) Proteolytic digestion of zona matrix
    (2) Hyperactivated motility (propulsion)

4.1.3 Zona Pellucida

4.1.4 Cortical and Zona Reactions (Polyspermy Block)

Cortical Reaction:

  Sperm fuses with oolemma (sperm-egg fusion: IZUMO1 + JUNO interaction)
       ↓
  Intracellular Ca²⁺ oscillations (waves from point of fusion — IP3-mediated)
  Calmodulin activation
       ↓
  Cortical granules (membrane-bound vesicles beneath oolemma) fuse with plasma membrane
       ↓
  Contents released into perivitelline space:
    • **Proteases** — cleave ZP2 and ZP3 → block further sperm binding
    • **Glycosidases** — modify ZP3 carbohydrates → inactivate sperm binding sites
    • **Cross-linking enzymes** — harden zona pellucida

Two Levels of Polyspermy Block:

4.1.5 Completion of Meiosis & Pronucleus Formation

  Sperm entry → Ca²⁺ oscillations activate oocyte
       ↓
  Meiosis II resumes (oocyte at metaphase II → anaphase II → telophase II)
       ↓
  Second polar body extruded (haploid)
  Oocyte now an **ovum** (haploid female pronucleus)
       ↓
  Sperm nucleus decondenses (protamines replaced by histones)
  **Male pronucleus** forms
       ↓
  Pronuclei migrate together, replicate DNA
  Nuclear membranes break down → **Syngamy**
  Chromosomes align on metaphase spindle
       ↓
  First mitotic division → **2-cell embryo** (~24–30 hours post-fertilisation)

4.1.6 Early Embryo Development (Pre-Implantation)

The first week of human development involves a series of cleavage divisions without overall growth in size (the embryo remains within the zona pellucida):

Key Molecular Events:

1. Embryonic Genome Activation (EGA): At the 4–8 cell stage, the embryonic genome becomes transcriptionally active. Before this, the embryo relies on maternal mRNA and proteins stored in the oocyte. Failure of EGA → developmental arrest (common in IVF).

2. Compaction (8–16 cell stage): Cells change from loosely adherent to tightly packed via E-cadherin (CDH1) mediated adhesion. Gap junctions form between cells. This is the first differentiation event — outer cells become trophoblast, inner cells become ICM.

3. Blastocyst Formation (Day 4–5): Na⁺/K⁺ ATPase pumps in trophectoderm cells pump Na⁺ into the intercellular space → water follows osmotically → blastocele (cavity) forms. The blastocyst expands within the zona pellucida.

4. Zona Hatching (Day 5–6): The blastocyst expands and thins the zona pellucida. Trophoblast cells secrete a zona-hatching enzyme (strypsin-like serine protease). The blastocyst escapes through a small hole. Assisted hatching in IVF can help embryos that fail to hatch spontaneously.

Clinical Correlation — Embryo Quality in IVF: - Day 2–3 transfer: grading based on cell number, fragmentation, symmetry - Day 5–6 (blastocyst) transfer: grading based on ICM + trophoblast quality - Blastocyst transfer has higher implantation rates → allows embryo selection - Extended culture to blastocyst carries risk of no embryo to transfer (if none survive)

Sex Determination: - Depends on which sperm fertilises the oocyte: X-bearing → 46,XX (female); Y-bearing → 46,XY (male) - The SRY gene on the Y chromosome triggers male development at ~6–8 weeks - In the absence of SRY → ovary develops (default pathway)

4.2 Implantation

4.2.1 The Implantation Window

Definition: The short period (days 6–10 post-ovulation, cycle days 20–24) when the endometrium is receptive to blastocyst implantation.

  Pre-receptive       Receptive       Non-receptive
  (Days 15–19)       (Days 20–24)     (Days 25+)
     |                    |                |
     |       IMPLANTATION WINDOW          |
     |<--------------------------------->|

Clinical correlation: IVF transfers are timed to the implantation window. If the window is displaced (e.g., due to ovarian stimulation with high oestrogen → premature secretory transformation → window shifts earlier), implantation fails.

4.2.2 Endometrial Receptivity

Molecular Markers of Receptivity:

The Embryo-Endometrial Dialogue:

  BLASTOCYST (Day 6–7)
       ↓ Secretes: hCG, IL-1, PGE2, LIF, HB-EGF
       ↓
  ENDOMETRIUM responds:
    • ↑ Vascular permeability
    • ↑ Chemokine secretion (attracts blastocyst)
    • ↑ Integrins (adhesion)
    • ↑ Decidualisation
    • Mucin (MUC1) cleared at attachment site
       ↓
  ENDOMETRIUM → Secretes: IGFBP-1, prolactin, LIF, cytokines
       ↓ Feedback to blastocyst
  TROPHOBLAST differentiation → invasion

4.2.3 Decidualisation

Definition: Transformation of endometrial stromal cells into specialised decidual cells, essential for implantation and placentation.

Initiation: Begins in mid-luteal phase (Day 21–22) independent of pregnancy. If pregnancy occurs, decidualisation continues; if not, the decidualised endometrium is shed.

Key Inducers: 1. Progesterone — binds PR-B in stromal cells → transcription of decidual genes 2. cAMP — local paracrine factor (generated by prostaglandins acting via EP receptors → ↑ cAMP) — synergistic with progesterone 3. Relaxin — from corpus luteum 4. hCG — from trophoblast (post-implantation)

Biochemical Hallmarks:

4.2.4 Trophoblast Invasion

Two Trophoblast Populations:

          BLASTOCYST
              |
        TROPHOBLAST
         /        \
   Villous       Extravillous
  (syncytio-      (cytotrophoblast,
    trophoblast)    invasive)
  - Exchange     - Spiral artery
  - hCG              remodelling
  - Barrier      - Anchoring villi

Extravillous Trophoblast (EVT) Invasion:

Result of Spiral Artery Remodelling:

  Original spiral artery:
    - Narrow lumen
    - Thick muscular wall
    - Responsive to vasoconstrictors
    - High resistance

  Remodelled spiral artery:
    - Wide lumen
    - No smooth muscle (replaced by fibrinoid)
    - No vasoconstrictor response
    - Low resistance, high flow
    - Continuous blood supply to intervillous space

Regulation of Trophoblast Invasion:

Clinical Correlation — Preeclampsia: - Defective spiral artery remodelling (inadequate EVT invasion) - Persistence of high-resistance, vasoreactive spiral arteries - Reduced utero-placental blood flow - Placental ischaemia → release of anti-angiogenic factors (sFlt-1, sEndoglin) → maternal endothelial dysfunction

4.3 Human Chorionic Gonadotrophin (hCG) — The Pregnancy Hormone

4.3.1 Structure and Production

4.3.2 Functions of hCG

1. Rescue of Corpus Luteum — Maintains progesterone production until placental takeover at ~8–10 weeks
2. TSH-like activity — Weakly stimulates thyroid (can cause gestational transient thyrotoxicosis)
3. Angiogenesis — Promotes uterine vascular remodelling
4. Immunomodulation — Suppresses maternal immune response to trophoblast
5. Trophoblast growth — Autocrine/paracrine role in trophoblast differentiation
6. Fetal testicular stimulation — In male fetus, acts like LH to stimulate Leydig cells → testosterone → male genital development

4.3.3 hCG Subtypes and Clinical Measurement

4.3.4 hCG Dynamics in Abnormal Pregnancy

5. Placental Endocrinology

5.1 The Placenta as an Endocrine Organ

The placenta is a transient endocrine organ that synthesises hormones otherwise produced by the hypothalamus, pituitary, ovary, and adrenal. It can do this because the syncytiotrophoblast is directly bathed in maternal blood and can secrete products directly into the maternal circulation.

5.2 The Feto-Placental Unit (Steroid Synthesis)

The placenta lacks CYP17 (17α-hydroxylase/17,20-lyase) — it cannot synthesise oestrogens from progesterone. It requires fetal precursors for oestrogen synthesis.

  MATERNAL COMPARTMENT            PLACENTA                   FETAL COMPARTMENT

                            ←———— Cholesterol —————       Fetal liver →
                                 ↓                              ↓
  Maternal DHEA-S -——→   DHEA-S (desulphated)    ←——   Fetal DHEA-S (fetal adrenal)
                              ↓                               ↓
                          Androstenedione               Fetal liver:
                              ↓ 16α-hydroxylation     16α-OH-DHEA-S
                          → 16α-OH-Androstenedione             ↓
                              ↓                          → To placenta
                          OESTRIOL (E3) ←——————————————

  OESTRADIOL (E2): from maternal + fetal androstenedione (no 16α-OH)
  OESTRONE (E1): from maternal androstenedione
  OESTRIOL (E3): requires **fetal 16α-hydroxylation** — 90% of pregnancy oestrogen!

Clinical Correlation: - Anencephaly (absent fetal adrenal): Fetal DHEA-S production ↓ → maternal oestriol very low - Smith-Lemli-Opitz syndrome: Defect in cholesterol synthesis → ↓ all steroids including oestriol - Placental sulphatase deficiency (X-linked ichthyosis): DHEA-S cannot be desulphated → ↓ oestriol but oestradiol normal → prolonged pregnancy, failure to go into labour - Maternal oestriol is used in triple/quad screening for Down syndrome (low oestriol → ↑ risk)

6. Maternal Physiological Adaptations to Pregnancy

6.1 Overview

Pregnancy induces profound changes in virtually every organ system, driven by: - Hormonal: Oestrogen, progesterone, hCG, hPL, cortisol, aldosterone, relaxin - Mechanical: Gravid uterus, diaphragmatic splinting, aortocaval compression - Haemodynamic: Blood volume expansion, cardiac output increase

6.2 Cardiovascular System

6.2.1 Key Changes — The "40% Rule"

Timeline: - CO increases by 10–15% by week 8 (before significant volume expansion — due to ↑ HR + ↓ SVR early) - Peaks at 24–28 weeks (~30–50% above non-pregnant) - Remains elevated until term - Drops dramatically in the first 2 weeks postpartum (loss of placenta → sudden ↓ SVR + ↓ blood volume)

Why ↑ CO in early pregnancy? 1. ↑ HR — oestrogen-mediated chronotropic effect on SA node 2. ↓ SVR — oestrogen + relaxin + PGI2 → vasodilation (especially renal, uterine, skin) 3. ↑ SV initially due to increased preload (↑ blood volume)

6.2.2 Clinical Implications

6.2.3 Physiologic Anaemia of Pregnancy

  Plasma volume:     ↑ 45%
  RBC mass:          ↑ 25%
        ↓
  **Hct drops from ~40% to ~33%**
  **Hb drops from ~13.5 g/dL to ~11.5 g/dL**
        ↓
  This is NOT true anaemia — it's dilutional
  (haemodilution improves uterine blood flow by reducing viscosity)

Diagnosis of true anaemia in pregnancy: - Hb < 11.0 g/dL (1st and 3rd trimester) or < 10.5 g/dL (2nd trimester) - Serum ferritin < 15 μg/L

6.3 Respiratory System

6.3.1 Key Changes

Why ↑ MV? - Progesterone directly stimulates chemoreceptors → ↑ respiratory drive - Progesterone also increases sensitivity to CO₂ → hyperventilation - Effect begins in 1st trimester (before mechanical changes)

6.3.2 Gas Exchange & Acid-Base

  ↑ MV → Alveolar CO₂ drops → PaCO₂ 30–32 mmHg (respiratory alkalosis)
       ↓
  Renal compensation: ↑ HCO₃⁻ excretion → HCO₃⁻ drops to 18–21 mEq/L
       ↓
  Net: mild alkalosis (pH 7.42–7.44)
       ↓
  O₂-Hb dissociation curve: shifts RIGHT (↑ 2,3-DPG) → facilitates O₂ unloading to fetus

Physiological Dyspnoea of Pregnancy: - ~75% of pregnant women report dyspnoea - Causes: ↑ awareness of breathing (progesterone effect), ↓ FRC (mechanical), ↑ ventilatory drive - Usually starts in 1st/2nd trimester BEFORE significant mechanical obstruction

Important exam point: The ↑ in MV is disproportionate to the ↑ in O₂ consumption (~20%) and CO₂ production. This is a progesterone-driven process, not a metabolic one.

6.4 Renal System

6.4.1 Key Changes

Mechanism of ↑ RBF + GFR: - Systemic vasodilation (oestrogen, relaxin → ↓ SVR → ↑ renal perfusion) - ↑ Cardiac output → ↑ renal plasma flow - Relaxin specifically ↑ endothelin type B receptor activation → NO → renal vasodilation - ↑ Plasma volume → ↑ preload

Clinical Implications:

6.5 Haematological System

6.5.1 Red Cell Changes

6.5.2 White Cell Changes

6.5.3 Coagulation Changes

The Pregnant Woman as a "Hypercoagulable State":

  ↑ Procoagulant factors + ↓ Protein S + ↓ fibrinolysis (↑ PAI) + ↑ stasis (veins)
       ↓
  5–10 × increased risk of VTE in pregnancy
  Risk highest in the postpartum period (especially first 6 weeks)

6.5.4 Iron, Folate & B12

6.6 Gastrointestinal System

6.7 Endocrine System

6.7.1 Thyroid

hCG and the Thyroid: - hCG shares the α-subunit with TSH and binds to the TSH receptor (weak agonist) - At the peak of hCG (~8–12 weeks), hCG can cause a transient ↓TSH and ↑FT4 - Gestational transient thyrotoxicosis: Nausea/vomiting, weight loss, palpitations. Self-limiting. NOT Graves' disease - Hyperemesis gravidarum: Severe vomiting associated with very high hCG → biochemical hyperthyroidism in ~60%

Trimester-Specific TSH Reference Ranges: - 1st trimester: 0.1–2.5 mIU/L - 2nd trimester: 0.2–3.0 mIU/L - 3rd trimester: 0.3–3.5 mIU/L

6.7.2 Adrenal

Physiological "Cushingoid" Appearance: - Striae gravidarum (not the same as pathological striae — they're purplish in Cushing's) - Central obesity distribution - All due to cortisol excess

6.7.3 Calcium Metabolism

6.7.4 Prolactin

7. Lactation

7.1 Stages of Lactation

7.2 Mammogenesis (Breast Development in Pregnancy)

  OESTROGEN (pregnancy)
       ↓
  Ductal elongation + branching
  ↑ Blood flow
  ↑ Fat deposition
  ↑ Pigmentation (areola)
  ↑ Prolactin receptors
       ↓
  PROGESTERONE (pregnancy)
       ↓
  Lobuloalveolar development
  (terminal ductal lobular units develop vacuoles)
       ↓
  PROLACTIN + hPL + GROWTH HORMONE (GH + placental GH variant)
       ↓
  Further alveolar differentiation
  Synthesis of milk proteins (casein, α-lactalbumin)
  ↑ Transport systems for glucose, amino acids, Ca²⁺

Hormonal Requirements for Mammogenesis:

7.3 Lactogenesis I (Colostrum Formation)

Timing: Starts at ~20 weeks of pregnancy.

Colostrum: - Yellowish, watery fluid - High in IgA (especially sIgA) - High in lactoferrin, lysozyme, oligosaccharides - Lower in lactose and fat than mature milk - Slight laxative effect (helps clear meconium)

Why doesn't milk secretion occur in pregnancy? - Progesterone blocks lactogenesis at high levels - Progesterone inhibits PRL stimulation of α-lactalbumin and suppresses secretory activation - Once placenta is delivered → progesterone withdrawal → full milk secretion

7.4 Lactogenesis II (Milk "Coming In")

Timing: 30–72 hours postpartum.

Trigger: Fall in progesterone after delivery of placenta + sustained high prolactin.

  PLACENTAL DELIVERY
       ↓
  Sudden drop in progesterone (and oestrogen)
       ↓
  Removal of progesterone block on lactogenesis
  Prolactin levels remain high (suckling→PRL release)
  Cortisol, insulin, T3 required permissively
       ↓
  Tight junctions between alveolar cells close
  ↓ paracellular pathway → strict transcellular secretion
       ↓
  Copious milk secretion
  (+++ lactose → water drawn osmotically → ↑ milk volume)
       ↓
  **Days 2–3:** Breasts engorged, warm, heavy ("milk in")

Electrolyte changes with tight junction closure:

  Open junctions (colostrum):
    Na⁺: ~45 mmol/L
    K⁺: ~15 mmol/L
    Cl⁻: ~45 mmol/L
    Lactose: ~130 mmol/L

  Closed junctions (mature milk):
    Na⁺: ~10 mmol/L
    K⁺: ~30 mmol/L
    Cl⁻: ~10 mmol/L
    Lactose: ~200 mmol/L

Clinical use: High Na⁺ in milk → suggests tight junctions open → inadequate lactation (e.g., retained placental fragment → ongoing progesterone → no tight junction closure).

7.5 The Milk Ejection Reflex

  SUCLKING (sensory input from nipple-areola)
       ↓
  Afferent arc (spinal cord → midbrain → hypothalamus)
       ↓
  Paraventricular nucleus (PVN) + Supraoptic nucleus (SON)
       ↓
  **OXYTOCIN** released from posterior pituitary
  (also PRF — prolactin-releasing factor — stimulates PRL)
       ↓
  Myoepithelial cells (smooth muscle-like) contract
  (surround alveoli + line ducts → "milk let-down")
       ↓
  Milk ejection into ducts → nipple → infant receives

Two phases of milk removal:

Psychogenic modulation: - Conditioned response: baby's cry, sight of baby can trigger oxytocin release - Stress, pain, anxiety can INHIBIT oxytocin release (adrenaline → vasoconstriction + α-adrenergic inhibition of oxytocin) - This is why breastfeeding requires relaxation and privacy — especially in first-time mothers

Clinical: Failure of milk ejection → baby gets foremilk but not hindmilk → unsatisfied baby, poor weight gain.

7.6 Prolactin

Suckling → Prolactin Release:

  Suckling → neural signal → hypothalamus
       ↓
  ↓ Dopamine (tonic inhibition removed)
  + ↑ PRF (prolactin-releasing factor: possibly VIP, TRH, oxytocin?)
       ↓
  Anterior pituitary releases PRL into circulation
       ↓
  Within 30 minutes: PRL peaks in blood (↑ 5–10 × baseline)
       ↓
  PRL acts on alveolar cells → milk synthesis

Prolactin and Amenorrhoea:

  PRL (high levels during breastfeeding)
       ↓
  ↑ dopamine turnover in hypothalamus (short loop feedback)
  ↓ GnRH pulse frequency
  ↓ LH pulsatility
  ↓ Kisspeptin expression in arcuate nucleus
       ↓
  **Lactational amenorrhoea** — anovulation
  Duration: variable (6–24 months depending on breastfeeding intensity)

  Clinical method: **Lactational Amenorrhoea Method (LAM)**
  Criteria: (1) <6 months postpartum, (2) fully breastfeeding, (3) amenorrhoeic
  Failure rate: <2% if all three criteria met

7.7 Breast Milk Composition

7.7.1 Colostrum vs Mature Milk

7.7.2 Foremilk vs Hindmilk

Key point: Infant must feed long enough to get hindmilk. Short feeds → only foremilk → inadequate calories, poor weight gain, lactose overload → green frothy stools.

7.7.3 Protective Factors in Breast Milk

Comparison: Human vs Cow's Milk:

8. Puberty

8.1 The HPG Axis — Maturation

During childhood, the HPG axis is quiescent due to: 1. Tonic inhibition by GABAergic neurons on GnRH neurons 2. Low amplitude GnRH pulses with minimal gonadotrophin release 3. High sensitivity to negative feedback — low oestradiol can suppress any GnRH activity

Reactivation at Puberty:

  Childhood (HPG suppressed)
       ↓
  ↑ KISS1 expression in arcuate nucleus
  ↓ GABAergic inhibition
  ↑ Glutamatergic stimulation
  ↓ Sensitivity to gonadal steroid negative feedback
       ↓
  GnRH pulse generator becomes active (NOCTURNAL first)
       ↓
  Sleep-entrained LH pulses → Oestradiol production → Secondary sex characteristics
       ↓
  Eventually 24-hour pulsatility → Menarche

8.2 Sequence of Pubertal Events

Normal Sequence (Tanner staging, mean ages):

Order of Events (Mnemonics: "The Pubescent Girl Matures After Menarche"):

  1. Thelarche (Breast budding)         — mean 10.5 years (range 8–13)
  2. Pubarche (Pubic hair)               — mean 11.0 years (range 8–14)
     + Adrenarche (Adrenal androgens)
  3. Growth spurt peaks                   — mean 12.0 years
  4. Menarche (First menstrual period)    — mean 12.8 years (range 10–16.5)
  5. Regular ovulation established        — 1–3 years after menarche

Key Facts: - Thelarche is usually the first sign of puberty (>85% of girls) - Menarche occurs at Tanner B4 (usually) - Peak height velocity occurs before menarche (typically 6–12 months before) - After menarche, girls grow an average of 5–7 cm more

8.3 Adrenarche

Definition: Maturation of the adrenal zona reticularis → ↑ production of dehydroepiandrosterone (DHEA), DHEA-S, and androstenedione.

Timing: - Begins at age 6–8 years (well before gonadarche) - DHEA-S rises from ~50 μg/dL (age 6) to ~200 μg/dL (age 12) - NOT mediated by ACTH alone — a specific "adrenarche factor" (? intra-adrenal changes in 17,20-lyase activity)

Clinical significance: - Adrenarche drives pubic hair (pubarche) and axillary hair - Premature adrenarche (<8 years) → investigate for congenital adrenal hyperplasia, adrenal tumour - Can be dissociated from gonadarche (normal variant: pubarche with no other pubertal changes)

8.4 Growth Spurt

Mechanism:

  OESTROGEN (from ovary)
       ↓
  ↑ GH secretion (↑ amplitude of GH pulses)
  ↑ IGF-1 production (liver + local)
       ↓
  ↑ Growth plate activity
  ↑ Epiphyseal fusion (eventually)
       ↓
  Growth spurt (~8–9 cm/year) over 2 years
  Total gain: ~25 cm during puberty

Oestrogen Paradox: - Oestrogen initially accelerates growth (↑ GH/IGF-1) - Oestrogen ultimately terminates growth (epiphyseal fusion via ERα on growth plate)

Gender difference: - Girls: earlier growth spurt (peak ~12 years), earlier epiphyseal fusion → shorter final height - Boys: later growth spurt (peak ~14 years), longer growth period → taller final height

8.5 Menarche

Definition: First menstrual period. Indicates sufficient oestrogen to build a proliferative endometrium and an ovulatory LH surge (or anovulatory withdrawal bleed).

Age range: - Normal: 10–16.5 years (mean 12.8) - Premature: <8 years (precocious puberty — requires investigation) - Delayed: >16 years (delayed puberty — requires investigation)

First cycles are often anovulatory (75% in first year, 50% by year 3, ~10% by year 5). Early cycles may be heavy (due to unopposed oestrogen → thick endometrium → anovulatory heavy bleed).

8.6 Premature (Precocious) Puberty

McCune-Albright Syndrome (Exam Favourite): - Triad: (1) Precocious puberty, (2) Café-au-lait spots (ragged "coast of Maine"), (3) Polyostotic fibrous dysplasia - Mechanism: Post-zygotic activating Gsα mutation (GNAS1) → constitutive cAMP production in ovary → oestrogen independent of FSH - Note: GnRH agonist does NOT work here (it's GnRH-independent) — treat with aromatase inhibitors or tamoxifen

8.7 Delayed Puberty

Constitutional Delay (CDGP): Most common cause of delayed puberty. Family history (+) in 50–80%. Variant of normal. Eventually catch up. Bone age < chronological age.

Kallmann Syndrome: - GnRH neurons fail to migrate from olfactory placode → GnRH deficiency + anosmia - X-linked (KAL1 mutation: anosmin-1) or autosomal dominant (FGFR1, PROKR2, PROK2) - Treatment: pulsatile GnRH or exogenous gonadotrophins

9. Menopause & Climacteric

9.1 Definitions

Average age of menopause: 51 years (range 45–55). Age is genetically determined (linked to BRCA1, FMR1 premutation, etc.)

9.2 Endocrine Changes

9.2.1 The Final Menstrual Cycle

  Age ~45–55
       ↓
  ↓↓ Primordial follicle pool (<1000 follicles)
       ↓
  ↓ Inhibin B (from small antral follicles)
       ↓
  **FSH rises** (loss of negative feedback from inhibin B)
  (FSH is the earliest hormonal sign of impending menopause)
       ↓
  Shorter follicular phase (accelerated follicle recruitment)
  → Shorter cycles (24–26 days initially)
       ↓
  ↓ Oestradiol — inconsistent (some cycles high! due to compensatory FSH)
  ↓ Inhibin A (from less competent corpus luteum)
       ↓
  Anovulatory cycles (eventually)
       ↓
  Skipped periods → no periods for 12 months → MENOPAUSE

9.2.2 Hormonal Profile by Stage

Key diagnostic criteria: - Perimenopause: FSH > 25 IU/L + cycle irregularity - Postmenopause: FSH > 40 IU/L + E2 < 100 pmol/L + 12 months amenorrhoea - POI: FSH > 40 IU/L + amenorrhoea for >4 months before age 40

9.2.3 Source of Oestrogen After Menopause

  ADRENAL CORTEX
       ↓
  DHEA-S + Androstenedione
       ↓ (17β-HSD, 5α-reductase, aromatase — in ADIPOSE TISSUE)
       ↓
  OESTRONE (E1) — weak oestrogen
  (not Oestradiol which is the main premenopausal oestrogen)

  Conversion: ↑ with BMI (more adipose = more aromatase)
  Hence: Obese women have higher circulating oestrone
         → lower FSH, less severe vasomotor symptoms
         → BUT higher risk of endometrial cancer (unopposed oestrone)

9.3 Clinical Features of Menopause

9.3.1 Vasomotor Symptoms (Hot Flushes)

Epidemiology: - ~75% of women experience hot flushes - Peak intensity: first 1–2 years postmenopause - After 5 years: ~50% still symptomatic - After 10 years: ~20% still symptomatic

Mechanism — KNDy Neuron Dysregulation:

  Oestrogen withdrawal
       ↓
  Hypothalamus loses oestrogen's inhibitory influence
       ↓
  KNDy neurons (kisspeptin, NKB, dynorphin) in arcuate nucleus
  become hyperactive/hypertrophied
       ↓
  Projections to **thermoregulatory centre** (preoptic area — POA)
       ↓
  Narrowing of the **thermoneutral zone**
  (normally ~0.4°C; in menopause → 0°C or even negative)
       ↓
  Even tiny temperature fluctuations trigger:
    - Heat dissipation mechanisms:
    - Cutaneous vasodilation (redness)
    - Sweating
    - Palpitations, anxiety
       ↓
  **Hot flush** — lasts 1–5 minutes

Treatments: - HRT — most effective (↑ oestrogen → restores thermoregulatory control) - NKB receptor antagonists (e.g., fezolinetant, elinzanetant) — novel non-hormonal agents that block NKB signalling → ↓ KNDy hyperactivity - Selective serotonin reuptake inhibitors (SSRIs/SNRIs — paroxetine, venlafaxine) — modulate hypothalamic thermoregulation - Clonidine — α₂-adrenergic agonist (reduces central noradrenergic hyperactivity) - Lifestyle — lower BMI, avoid triggers (hot drinks, caffeine, alcohol)

9.3.2 Urogenital Atrophy

Treatment: - Topical vaginal oestrogen (cream, tablet, ring) — very effective, minimal systemic absorption - Vaginal moisturisers + lubricants — for mild symptoms

9.3.3 Bone Loss

Mechanism:

  ↓ Oestradiol
       ↓
  ↑ RANKL (on osteoblasts)
  ↓ OPG (osteoprotegerin) — decoy receptor for RANKL
       ↓
  ↑ RANKL/OPG ratio → ↑ Osteoclast activation
       ↓
  Accelerated bone resorption
       ↓
  2–5% bone loss per year in first 5–10 postmenopause
  (vs 0.5% in premenopause)
  **50% of lifetime bone loss occurs in first 10 postmenopausal years**

Clinical effects: - Vertebral fractures (Colles', hip, vertebral compression) - Loss of height, kyphosis (dowager's hump) - Osteoporotic fractures most common after age 65+

Screening: - DEXA scan at menopause if risk factors (BMI < 20, family history, steroid use, smoking, early menopause) - T-score ≤ -2.5 = osteoporosis

9.3.4 Cardiovascular Implications

Before menopause, women have lower CVD risk than men. After menopause, risk equalises.

Note: While HRT improves lipid profile (↑ HDL, ↓ LDL), the effect on CVD outcomes is complex (timing hypothesis: HRT may be protective if started near menopause but harmful if started >10 years after).

9.4 HRT (Hormone Replacement Therapy) — Principles

9.4.1 Rationale

Replace the oestrogen deficit. Progesterone (progestogen) is added for anyone with a uterus to prevent endometrial hyperplasia/cancer from unopposed oestrogen.

9.4.2 Regimens

9.4.3 Routes of Administration

9.4.4 Risks vs Benefits

10. Appendix: Exam Mnemonics & Key Numbers

10.1 Key Numbers to Memorise

10.2 Mnemonics

Order of Puberty (The Pubescent Girl Matures After Menarche): - Thelarche - Pubarche - Growth spurt - Menarche - Adult ovulatory cycles - Mature fertility

Tanner Staging (Breast) — "Buds, Bumps, Breasts, Beyond": - B1: Prepubertal - B2: Breast bud - B3: Breast elevated (no areolar separation) - B4: Areolar mound (secondary) - B5: Mature (areolar recession)

Endometrial Cycle — "Mary Pets Some Insects": - Menstrual - Proliferative - Secretory - Ischaemic

Factors increasing in pregnancy — "Pregnant Women Always Feel Heavy": - Plasma volume ↑ - White cells ↑ - Aldosterone ↑ - Fibrinogen ↑ - Heart rate ↑ - Hormones (E2, P4, hCG)

Factors decreasing in pregnancy: - Hb, Hct, SVR, BP (2nd trimester), FRC, creatinine, urea, albumin, PaCO₂

KNDy Neuron Triad: - Kisspeptin — Stimulates GnRH - Neurokinin B — Stimulates kisspeptin release - Dynorphin — Inhibits KNDy neurons (slows GnRH)

The 40s Rule of Pregnancy Adaptations: - CO +40% - Plasma volume +40% - SVR -40% - MV +40% - GFR +40–50% - Blood volume +40–50%

2-Cell 2-Gonadotrophin Mnemonic — "Theca + LH = Androgens; Granulosa + FSH = Aromatise": - Theca: LH → Androstenedione - Granulosa: FSH → Aromatase → Oestradiol

Hormones that ↑ in Menopause: - FSH - LH - GnRH - E1 (relatively)

• The rest of oestrogen, progesterone, inhibin, AMH — all ↓

10.3 Clinical Vignette Patterns (MRCOG Style)

10.4 Summary: Steroidogenesis Quick Reference

                        CHOLESTEROL
                            ↓ (CYP11A1/SCC)
                       PREGNENOLONE
                    ↗           ↘ (3β-HSD)
             17α-OH-Preg           PROGESTERONE
                  ↓ (CYP17)             ↓ (CYP17)
             17α-OH-Preg           → 17α-OH-Progesterone (17-OHP)
                  ↓ (CYP17-17,20-lyase)
          DEHYDROEPIANDROSTERONE (DHEA)
                  ↓ (3β-HSD)
                ANDROSTENEDIONE
              ↗           ↘ (17β-HSD)
        TESTOSTERONE      OESTRONE (E1)
          ↘                   ↓ (17β-HSD)
        (5α-reductase)    OESTRADIOL (E2)
               ↓
          DHT (dihydrotestosterone - potent androgen)

References & Further Reading

1. Johnson MH. Essential Reproduction. 8th ed. Wiley-Blackwell.
2. Llewellyn-Jones D, Oats JN. Fundamentals of Obstetrics and Gynaecology. 10th ed.
3. Cunningham F, et al. Williams Obstetrics. 26th ed. McGraw-Hill.
4. Chandra R, Ganeshan B, et al. MRCOG Part 1: A Complete Guide. RCOG Press.
5. RCOG. MRCOG Part 1 Syllabus. Royal College of Obstetricians and Gynaecologists.
6. Gardner DG, Shoback D. Greenspan's Basic & Clinical Endocrinology. 10th ed.
7. Hall JE. Guyton and Hall Textbook of Medical Physiology. 14th ed.
8. Speroff L, Fritz MA. Clinical Gynecologic Endocrinology and Infertility. 9th ed.
9. Mihm M, Gangooly S, Muttukrishna S. The normal menstrual cycle in women. Anim Reprod Sci. 2011;124(3-4):229-36.

End of Document Total: ~22,500 words over 9 major sections Last updated: May 2026