A woman at 32 weeks gestation has a GFR of 165 mL/min (versus her pre-pregnancy value of 110 mL/min). The serum creatinine is 0.5 mg/dL. Why is GFR elevated in pregnancy, and what is the clinical significance?

• A Elevated GFR due to progesterone increasing glomerular permeability selectively to creatinine
• B The elevated GFR is due to increased tubular secretion rather than true filtration change
• C GFR rises in pregnancy due to anatomical hypertrophy of nephrons under hCG stimulation
• D Increased renal blood flow due to plasma volume expansion and renal vasodilation; creatinine and urea appear falsely low, requiring adjusted reference ranges ✓

Explanation

In pregnancy, plasma volume increases by ~45–50%, cardiac output by ~40%, and renal blood flow by ~60–80%. Hormones (relaxin, progesterone, estrogen) cause renal afferent arteriolar vasodilation, raising GFR by 50–60% above non-pregnant values. Consequently, serum creatinine (normal 0.5–0.7 mg/dL in pregnancy vs. 0.6–1.0 mg/dL non-pregnant), urea, and uric acid are all lower than normal — a serum creatinine of 1.0 mg/dL may indicate significant renal impairment in a pregnant woman even though it appears 'normal.'

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

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