Despite the fall in the incidence of cardiac disease during pregnancy over the last 25 years, when a cardiac surgery is required the risk is obviously greater as two lives are at risk. The risk to the mother is now similar to that for non-pregnant female patient but the fetal mortality remains high. Cardiac operation is ill advised except in extreme emergencies the first two trimesters, as the incidence of teratogenesis is high. During the third trimester, with improvements in the outcome for premature infants with modern neonatal intensive care, delivery of the child immediately before commencing cardiopulmonary bypass is a safe option. If this is inappropriate, high flow, high pressure, normothermic bypass for as brief period as possible should be used. However, although it has theoretic advantages, the benefit of pulsatile perfusion is unproven. The fetal response to cardiopulmonary bypass is bradycardia thought to be due to hypoperfusion secondary to uterine contractions, and this dysrrhythmia is reversible by increasing the perfusion rate. Fetal heart rate monitoring is therefore essential to allow these manipulations. The response of fetoplacental unit is more complex, comprising two elements: an early vasoactive response is due to prostaglandin synthesis, whereas a more profound late acidosis appears to be related to fetal stress response. Finally, uterine contractions occur in response to bypass, possibly due to a dilutional effect from the stabilizing influence of progesterone. Various techniques to modify this include the administration of progesterone, beta2-agonists, and intravenous alcohol, all with some effect. Uterine monitoring is essential to allow early control of these contractions, as they are associated with significant fetal loss.