At term, a woman's cardiac output is approximately 40–50% higher than her pre-pregnancy baseline. Which change makes the greatest single contribution to this increase?

• A Increased stroke volume driven by expanded plasma volume and reduced SVR ✓
• B Increased heart rate (~20 beats/min increase)
• C AV fistula-like effect of the low-resistance uteroplacental circulation
• D Progesterone-mediated positive inotropic effect on myocardium

Explanation

The increase in cardiac output during pregnancy is attributable to both increased stroke volume (the dominant early contribution, mediated by ~50% plasma volume expansion and reduced systemic vascular resistance due to progesterone/relaxin-driven vasodilation) and increased heart rate (~15–20 bpm). The stroke volume increase begins early in the first trimester and peaks around mid-gestation, while the heart rate contribution is sustained. Progesterone has no direct positive inotropic effect.

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

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