What drives the increase in cardiac output in early versus late pregnancy?

Cardiac output = stroke volume x heart rate. In early pregnancy the rise is mainly due to increased stroke volume (which increases up to ~16 weeks then plateaus).

In late pregnancy, heart rate is the major contributor (it rises slightly to ~12 weeks, then increases again after ~32 weeks and stays elevated until delivery), as stroke volume slightly reduces.

Clinical relevance. Understanding the stroke-volume-then-heart-rate basis explains the physiological tachycardia and flow murmur of pregnancy.

More Detail

Determinants of the cardiac output rise

The relative contributions of stroke volume and heart rate shift across pregnancy.

Stroke volume

• Increases up to ~16 weeks then plateaus, and is slightly reduced later.
• Driven by increased preload (plasma volume up 50%) and increased contractility/compliance.

Heart rate

• Slightly increases up to ~12 weeks then plateaus; rises again after ~32 weeks and remains elevated until delivery (by ~10-20 bpm overall).

Early vs late

• Early pregnancy - the increased cardiac output is mainly due to the rise in stroke volume.
• Late pregnancy - heart rate is the major factor (as stroke volume falls slightly).

Underlying changes

Preload increases (plasma volume +50%), afterload reduces (vasodilation lowers peripheral resistance), and contractility/compliance increase - together raising cardiac output.