A 28-year-old woman with known rheumatic mitral stenosis (mitral valve area 1.0 cm²) becomes pregnant at 8 weeks. She is in NYHA Class II. Which statement about her hemodynamic risk during pregnancy is MOST accurate?

• A The peak risk period is at 32–34 weeks when cardiac output reaches its maximum of approximately 45% above baseline
• B The highest risk period is at delivery and immediate postpartum due to autotransfusion of 300–500 mL from the uterus
• C Rheumatic mitral stenosis in pregnancy carries greatest risk at 20–24 weeks due to maximal plasma volume expansion without compensatory cardiac adaptation
• D Maternal tachycardia of >120 bpm during labor is the single most dangerous hemodynamic change for mitral stenosis ✓

Explanation

In rheumatic mitral stenosis, the fixed obstruction across the mitral valve means that any increase in heart rate shortens diastolic filling time — the critical period for left ventricular filling through the stenotic valve. Tachycardia (>120 bpm) during labor contractions, anxiety, or pain dramatically reduces diastolic filling time, causing acute elevation of left atrial and pulmonary venous pressure, precipitating acute pulmonary edema. This is why rate control (beta-blockers) is central to managing mitral stenosis in pregnancy. While cardiac output peaks at 28–32 weeks (not 32–34, ruling out A), and autotransfusion at delivery (B) is a risk, tachycardia is the single most dangerous acute hemodynamic event. The 20–24 week period (C) is the second trimester peak, relevant for cardiomyopathy.

Reference: Williams Obstetrics, 26th ed.

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Written and medically reviewed by the StethoPrep medical team.