Physiology of Pregnancy

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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).

Human fertilization, cleavage, blastocyst, and implantation

⭐ 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. Key hormonal changes during pregnancy

⭐ 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.

Practice Questions: Physiology of Pregnancy

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What is the typical time between fertilization and implantation?

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Flashcards: Physiology of Pregnancy

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Does glucose cross the placenta? What about insulin? _____

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Does glucose cross the placenta? What about insulin? _____

Glucose does

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