During normal pregnancy, maternal plasma volume expands by approximately what percentage by the third trimester, and which hormone is primarily responsible?

• A 15–20%; progesterone directly causing sodium retention in the proximal tubule
• B 25–30%; hCG stimulating aldosterone-independent sodium retention
• C 50–60%; ANP released from the stretched atria driving further plasma expansion
• D 45–50%; aldosterone stimulated by the oestrogen-driven rise in renin-angiotensin system ✓

Explanation

Maternal plasma volume increases by 40–50% (approximately 1,200 mL) above the non-pregnant state by 28–34 weeks gestation. This is primarily driven by the activation of the renin-angiotensin-aldosterone system (RAAS). Oestrogen stimulates hepatic renin substrate (angiotensinogen) production, amplifying angiotensin II and aldosterone secretion. Aldosterone causes sodium and water retention in the distal nephron, expanding extracellular and plasma volume. The disproportionate plasma expansion relative to red cell mass increase (~20–30%) creates the physiological anaemia of pregnancy (haemodilution). Options B, C, and D give incorrect percentages or incorrect primary hormones.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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