A 28-week-pregnant woman has her cardiac output measured. Compared to her pre-pregnancy baseline, which combination of changes is MOST accurate?

• A Cardiac output +40–50%; heart rate +15–20 bpm; stroke volume increased; SVR decreased by ~20% ✓
• B Cardiac output +10–15%; heart rate unchanged; stroke volume increased due to hypervolemia
• C Cardiac output +40–50%; heart rate unchanged; achieved entirely by stroke volume increase
• D Cardiac output −10%; SVR decreased by 40%; compensated by reflexive tachycardia

Explanation

By the third trimester, cardiac output increases by 40–50% above baseline, rising from approximately 5 to 7–8 L/min. This is achieved by both an increase in heart rate (15–20 bpm) and stroke volume (due to increased plasma volume and preload). Systemic vascular resistance decreases by approximately 20% due to placental arteriovenous shunting, progesterone-mediated vasodilation, and increased nitric oxide production. Option B underestimates the magnitude. Option C incorrectly implies heart rate does not change. Option D depicts a fall in cardiac output, which would indicate pathology.

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

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Written and medically reviewed by the StethoPrep medical team.