During pregnancy, the maternal kidney increases creatinine clearance. Which mechanism is primarily responsible for the rise in GFR during pregnancy?

• A Progesterone dilates the renal pelvis and calyx, reducing back-pressure and passively increasing filtration
• B Relaxin-mediated renal vasodilation increases renal plasma flow, reducing afferent arteriolar resistance and increasing GFR by ~50% ✓
• C hCG directly acts on glomerular mesangial cells to increase filtration surface area
• D Increased cardiac output alone accounts for the rise in GFR through increased renal perfusion pressure

Explanation

GFR rises by approximately 40–50% in pregnancy, beginning in the first trimester. The key mediator is relaxin, a hormone produced by the corpus luteum and placenta. Relaxin stimulates endothelin B receptors and eNOS in the renal vasculature, increasing nitric oxide production and causing significant afferent arteriolar vasodilation. This reduces renal vascular resistance, increases renal plasma flow, and consequently raises GFR. Serum creatinine and BUN fall to about 0.5–0.8 mg/dL and 8–10 mg/dL respectively, so 'normal' non-pregnant values can indicate impaired renal function during pregnancy. While cardiac output increases, this alone does not account for the GFR rise; renal vasodilation is the critical mechanism. Options B–D are incorrect or incomplete.

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

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Written and medically reviewed by the StethoPrep medical team.