OG7.1 | Maternal Physiological Changes in Pregnancy — Summary & Reflection

KEY TAKEAWAYS

Maternal physiological changes in pregnancy represent purposeful, system-wide adaptations to meet feto-placental demands. Key points to retain:

Cardiovascular: CO ↑30–50% (SV + HR); SVR ↓20–30%; BP falls in mid-pregnancy (nadir 20–24 weeks); physiological systolic murmur is normal; aortocaval compression after 20 weeks in supine position — use left lateral tilt.

Haematological: Plasma volume ↑~50%, RBC mass ↑~25% → dilutional anaemia (Hb threshold: <11 g/dL WHO). Iron requirement ↑ (27 mg/day); hypercoagulability (↑fibrinogen, ↑VII/VIII/X, ↓protein S) → 5–10× VTE risk.

Respiratory: TV ↑40%, minute ventilation ↑30–50% (progesterone-driven); FRC ↓20%; compensated respiratory alkalosis (PaCO₂ ~30–32 mmHg, HCO₃⁻ ~18–22 mEq/L); rapid desaturation risk.

Renal: GFR ↑50% → creatinine normally 0.4–0.6 mg/dL; physiological glycosuria (do not diagnose GDM from dipstick); ureteric dilation → UTI risk.

GI/Endocrine: Nausea/vomiting (hCG); GORD (progesterone ↓LOS); constipation; insulin resistance (hPL) → GDM risk; weight gain 11.5–16 kg (normal BMI).

Hormonal drivers: Progesterone (smooth muscle relaxation, respiratory stimulation); oestrogen (vascular growth, coagulation); hCG (NVP, corpus luteum); hPL (insulin resistance); relaxin (joints, renal vasodilation); aldosterone (volume expansion).

Clinical rule: Always apply pregnancy-specific reference ranges. The most dangerous errors arise from using non-pregnant normal values for creatinine, haemoglobin, and blood gases.

REFLECT

Consider a scenario from clinical practice: a junior doctor reviews a pregnant woman's blood results and, not knowing pregnancy-specific ranges, recommends urgent investigation for anaemia (Hb 10.8 g/dL at 28 weeks) and possible renal disease (creatinine 0.5 mg/dL) — one is unnecessary, the other misses the opposite problem. Reflect on how the physiological changes you have learned in this module would change your clinical reasoning if you were in the same situation. Which single change across all systems do you find most clinically impactful — and why? How would you explain to a patient why her 'low' haemoglobin or 'low' blood pressure is, in fact, a sign of a healthy pregnancy?