Why does pre-existing heart disease cause problems in pregnancy?

Pregnancy demands a large rise in cardiac output (30-40% by ~32 weeks, up to 80% in labour, and an 80% surge immediately postpartum from autotransfusion).

A woman with pre-existing heart disease cannot increase her cardiac output as required, so she is at risk of decompensation and heart failure at these points of peak demand - especially in labour and the early postpartum period.

Clinical relevance. Knowing the peaks of cardiac demand (labour, postpartum) explains when women with heart disease are most at risk.

More Detail

Heart disease and the demands of pregnancy

Pregnancy is a high-output state that a diseased heart may not tolerate.

The haemodynamic load

• Cardiac output rises 30-40%, mostly by ~32 weeks.
• During labour, cardiac output increases up to 80% (pain, contractions, pushing).
• Immediately postpartum, an ~80% surge in cardiac output occurs from autotransfusion (uterine contraction returns blood to the circulation) and relief of caval compression.

Why disease causes problems

• A woman with pre-existing heart disease cannot increase cardiac output to meet these demands.
• This risks decompensation and heart failure, particularly at the peaks of demand - labour and the early postpartum period.
• Fixed-output lesions (e.g. severe mitral stenosis, pulmonary hypertension) are especially dangerous.