Pregnancy and Lactation Physiology
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Direct answer
Human chorionic gonadotropin, secreted by the syncytiotrophoblast and detectable about 8-11 days after ovulation, doubles every 48-72 hours and peaks at 8-10 weeks near 100,000 IU/L, sustaining the corpus luteum until the luteo-placental shift at 8-10 weeks. Maternal adaptations follow a fixed script: blood volume rises 40-50 per cent to about 5 litres (plasma outpacing red cells, producing the physiological anaemia of pregnancy with haemoglobin near 11 g/dL), cardiac output climbs 30-50 per cent to 6-7 L/min while arterial pressure dips to its nadir around 24 weeks, glomerular filtration rate rises 40-50 per cent so creatinine falls below 0.8 mg/dL, and progesterone drives a 40 per cent rise in minute ventilation leaving a mild respiratory alkalosis with PaCO2 near 27-32 mm Hg. The fetus runs its own circulation — foramen ovale and ductus arteriosus bypassing the lungs, ductus venosus bypassing the liver, umbilical venous PO2 only 30 mm Hg offset by fetal haemoglobin's P50 of 19. Lactation pairs prolactin (synthesis) with oxytocin (let-down), yielding about 850 mL of milk daily after colostrum's first 2-3 days.
What you must remember
- hCG numbers: detection 8-11 days post-ovulation, doubling every 48-72 hours, peak 8-10 weeks at about 100,000 IU/L, then decline; alpha subunit shared with LH, FSH and TSH; the basis of urine tests and molar surveillance.
- Endocrine handover: corpus luteum maintains pregnancy until 8-10 weeks (luteo-placental shift); oestriol dominates later pregnancy via the fetoplacental unit — fetal adrenal DHEA-S converted by placenta into a fetal well-being marker.
- Cardiovascular numbers: blood volume up 40-50 per cent (plasma 50, red cell 20-30); cardiac output up 30-50 per cent, heart rate up 15-20 beats; blood pressure nadirs mid-trimester; supine hypotensive syndrome from aortocaval compression.
- Renal and respiratory: GFR up 40-50 per cent (creatinine under 0.8); tidal volume up 40 per cent, PaCO2 27-32 mm Hg — a compensated respiratory alkalosis the exam quotes.
- Fetal circulation logic: umbilical vein PO2 30 mm Hg, saturation about 80 per cent — the best-oxygenated blood; fetal haemoglobin P50 about 19 mm Hg; right ventricle dominance with only 10-15 per cent of output through the lungs; ductus arteriosus kept open by prostaglandin E2, closed by indomethacin.
- Placental transfer: IgG crosses actively (passive neonatal immunity), IgM does not; gases cross by simple diffusion; the placenta performs respiratory, nutritive, excretory and endocrine work for the fetus.
- Lactation kit: prolactin from the suckling reflex (dopamine inhibits, TRH stimulates), oxytocin contracts myoepithelial cells for let-down (Ferguson reflex); colostrum rich in protein and secretory IgA, low in fat; mature milk about 850 mL/day, 88 per cent water; lactational amenorrhoea is over 98 per cent protective for six months with exclusive feeding — the LAM criterion of India's family-planning programme.
Three trimesters of numbers
First trimester: hCG peaks and falls, the corpus luteum retires at 8-10 weeks, and organogenesis closes most windows by week 10 — the teratogenic danger zone. Second trimester: plasma volume expands fastest and blood pressure nadirs around 24 weeks — chronic hypertension may masquerade as normal now and unmask later. Third trimester: the supine uterus compresses the inferior vena cava, relieved by left lateral tilt. In the puerperium, placental delivery removes steroid inhibition of prolactin, and suckling maintains both prolactin (synthesis) and oxytocin (ejection, involution). Colostrum's secretory IgA coats the neonatal gut, and exclusive breastfeeding for six months remains the WHO and national recommendation.
Where students slip
First slip: calling pregnancy anaemia pathological. Plasma expanding 50 per cent against red cells 20-30 per cent dilutes haemoglobin to about 11 g/dL by design — anaemia in pregnancy is diagnosed below 11 g/dL, not below the generic 12. Second, the pressure story: cardiac output is up 30-50 per cent yet blood pressure is down in mid-pregnancy — resistance has fallen more than output has risen; pre-eclampsia inverts this after 20 weeks. Third, fetal oxygen: the fetus thrives at a PO2 of 30 mm Hg because P50 19 haemoglobin stays saturated where adult haemoglobin unloads — applying adult curves misjudges the neonatal transition, when the ductus closes and pulmonary resistance falls at the first breath.
Frequently asked questions
When does hCG peak and why does it decline?
It peaks at 8-10 weeks near 100,000 IU/L, then falls as the placenta assumes progesterone production, stabilising at a lower plateau.
Why does pregnancy cause a physiological anaemia?
Plasma volume expands about 50 per cent while red cell mass rises only 20-30 per cent, diluting haemoglobin to around 11 g/dL.
How does the fetus achieve adequate oxygenation at low PO2?
Umbilical venous PO2 is only about 30 mm Hg, but fetal haemoglobin's P50 of 19 mm Hg reaches 80 per cent saturation where adult haemoglobin would unload.
What keeps the ductus arteriosus open and what closes it?
Prostaglandin E2 keeps it open; falling PGE2 at birth plus rising oxygen tension close it, and indomethacin closes it pharmacologically.
Which hormones govern milk synthesis and ejection?
Prolactin drives synthesis (inhibited by dopamine, stimulated by suckling), while oxytocin contracts myoepithelial cells for let-down and uterine involution.