Physiology of Pregnancy

Fertilization & Implantation - The Great Beginning

• Fertilization: Ampulla of fallopian tube.
  • Sperm penetrates corona radiata, zona pellucida.
  • Requires:
    • Capacitation: Final sperm maturation.
    • Acrosome reaction: Enzymes (hyaluronidase, acrosin) for zona penetration.
  • Cortical reaction: Oocyte block to polyspermy. Forms Zygote ($2n$).
• Implantation: Blastocyst (Day 5-6) embeds in endometrium (decidual reaction).
  • Window: Day 6-10 post-ovulation.
  • Site: Posterior superior uterine wall.
  • Steps: Apposition, Adhesion, Invasion (📌 API).
  • Trophoblast differentiates:
    • Cytotrophoblast (inner).
    • Syncytiotrophoblast (outer, invasive, secretes hCG).

⭐ hCG (from syncytiotrophoblast) detectable in serum 8-9 days post-ovulation (1 day post-implantation); earliest basis of pregnancy tests.

Placental Physiology - The Baby's Lifeline

• Key Functions:
  • Gas Exchange: O₂ to fetus (HbF ↑ affinity), CO₂ to mother. Facilitated by Bohr & Haldane effects.
  • Nutrient Transfer:
    • Glucose: Facilitated diffusion (GLUT).
    • Amino Acids: Active transport.
    • IgG: Pinocytosis (passive immunity).
    • Fatty acids, vitamins, electrolytes.
  • Waste Excretion: Urea, creatinine from fetus.
  • Endocrine:
    • hCG: Maintains corpus luteum; basis of pregnancy tests.
    • hPL (hCS): ↑ maternal insulin resistance → ↑ fetal glucose.
    • Progesterone: "Pregnancy hormone"; quiets uterus.
    • Estrogens: Uterine/breast development.
  • Barrier (Limited): Protects but permeable to some drugs/infections (e.g., TORCH).

⭐ hCG (Human Chorionic Gonadotropin): Produced by syncytiotrophoblast, peaks at 8-10 weeks, maintains corpus luteum to produce progesterone until placenta takes over (luteo-placental shift).

Maternal Adaptations - The Body's Big Shift

• Cardiovascular: ↑ CO (30-50%), ↑ HR (15-20 bpm). ↓ SVR, ↓ BP (nadir 2nd tri). Supine hypotension. Physiological anemia (↑ plasma vol 40-50% > ↑ RBC mass 20-30%).
• Respiratory: ↑ TV, ↑ Minute vent. ↓ FRC (20-30%). Compensated respiratory alkalosis ($PCO_2$ ↓ ~30 mmHg).
• Renal: ↑ RPF & ↑ GFR (~50%). ↓ BUN, Creatinine. Mild glycosuria/proteinuria.
• Hematological: Leukocytosis. Hypercoagulable state (↑ DVT/PE risk).
• Metabolic: Insulin resistance (hPL effect) → diabetogenic state.
• GI: ↓ Motility → constipation, GERD. Nausea/vomiting.

⭐ Plasma volume increases by 40-50%, while RBC mass increases by only 20-30%, leading to physiological hemodilution and a fall in Hb concentration.

Hormonal Orchestra - Pregnancy's Conductors

• hCG (Human Chorionic Gonadotropin):
  • By Syncytiotrophoblast.
  • Maintains corpus luteum → progesterone.
  • Peaks: 8-10 weeks. Pregnancy test basis.
• Progesterone: "Pro-gestation" hormone.
  • Source: Corpus luteum (→ placenta ~7-10 wks).
  • Actions: ↓ uterine contractility, decidual support, breast prep.
• Estrogens (Mainly Estriol - E3):
  • Source: Corpus luteum → feto-placental unit.
  • Actions: Uterine/breast growth, ↑ prolactin, ↑ uteroplacental flow.
• hPL (Human Placental Lactogen) / hCS:
  • Source: Placenta.
  • Actions: Anti-insulin (↑ glucose for fetus), lipolysis, breast dev.
• Relaxin:
  • Source: Corpus luteum, placenta.
  • Actions: Cervical ripening, relaxes ligaments.

⭐ hCG levels double approx. every 48-72 hours in early viable intrauterine pregnancy.

High‑Yield Points - ⚡ Biggest Takeaways

• hCG: Doubles q48-72h in early pregnancy, peaks 8-10 wks, maintains corpus luteum.
• Progesterone: From corpus luteum/placenta, ensures uterine quiescence, supports endometrium.
• Estrogen (Estriol): Uterine/breast growth, ↑ clotting factors; estriol indicates fetal well-being.
• hPL: Induces maternal insulin resistance (↑ fetal glucose supply), promotes lipolysis.
• Cardiovascular: ↑ CO, ↓ SVR, marked ↑ plasma volume (physiological anemia).
• Respiratory: ↑ Tidal volume, ↑ minute ventilation, chronic compensated respiratory alkalosis.
• Renal: ↑ GFR, ↑ RPF; physiological glucosuria can be common.