Describe the hormones of the placenta and the maternal physiological changes during pregnancy. Add a note on parturition.

Fertilisation and Implantation

Ovulation → oocyte picked up by the fimbriae → Fertilisation in the ampulla of the uterine tube → Zygote → morula (day 3) → Blastocyst (day 5) → Implantation on day 6–7 in the upper posterior uterine wall

• The ovum is fertilisable for 12–24 hours; sperm survive 48–72 hours
• Zona reaction after the first sperm enters prevents polyspermy

Placental Hormones

• Luteo-placental shift — the corpus luteum supports the pregnancy for the first 6–8 weeks, after which the placenta takes over. Removing the ovary before this causes abortion; after it, the pregnancy continues

Maternal Cardiovascular Changes

Other Systemic Changes

• Respiratory — tidal volume ↑ 40%, so alveolar ventilation rises; PaCO2 falls to 32 mmHg → a mild respiratory alkalosis which aids placental CO2 transfer
• Renal — GFR ↑ 50% → serum urea and creatinine fall; the lowered threshold causes benign glycosuria
• Metabolic — weight gain 10–12 kg; BMR ↑ 15–20%; insulin resistance from hPL, unmasking gestational diabetes
• Haematological — a hypercoagulable state (↑ fibrinogen and clotting factors) → protects against postpartum haemorrhage but raises the risk of thromboembolism
• GI — delayed emptying, relaxed lower oesophageal sphincter → heartburn; constipation

Functions of the Placenta

• IgG crosses the placenta → gives the newborn passive immunity for 3–6 months; it is also the route of anti-D in erythroblastosis fetalis
• Torch organisms cross and cause congenital infection

Fetal Circulation Adaptations

• Fetal haemoglobin (HbF) — higher O2 affinity, P50 19 mmHg, so it extracts oxygen from maternal blood
• Double Bohr effect — CO2 passing to the mother acidifies her blood (right shift, releases O2) while alkalinising fetal blood (left shift, takes up O2)
• Double Haldane effect — the same arrangement aids CO2 transfer
• Fetal haemoglobin concentration is high (18–20 g/dL)
• Umbilical vein blood is only 80% saturated — the fetus lives in "Mount Everest in utero" conditions

Parturition

Term reached → ↑ Oestrogen : progesterone ratio → ↑ Oxytocin receptors + ↑ gap junctions in myometrium → Contractions begin; head stretches the cervix → ferguson reflex → ↑ oxytocin → Stronger contractions → positive feedback → delivery

• Prostaglandins (PGE2, PGF2α) ripen the cervix and are essential to labour
• Three stages — dilatation, expulsion, and placental separation

Applied Aspects

• Ectopic pregnancy — commonest in the ampulla; a surgical emergency if it ruptures
• Pre-eclampsia — hypertension with proteinuria after 20 weeks; defective trophoblastic invasion of the spiral arteries
• Supine hypotension syndrome — the gravid uterus compresses the inferior vena cava → ↓ venous return; relieved by the left lateral position
• Physiological anaemia is dilutional, not true iron deficiency — but iron and folate are supplemented routinely in India
• Gestational diabetes — hPL-driven insulin resistance; screened at 24–28 weeks; raises the lifetime risk of type 2 diabetes
• Rh isoimmunisation — prevented by anti-D within 72 hours of delivery or abortion
• Placenta praevia and abruption — the two great causes of antepartum haemorrhage