Thromboembolic disorders are still a serious problem in pregnancy and anticoagulants have an important part to play in both treatment and prevention. Warfarin is the most convenient drug to give but can cause maternal and fetal bleeding problems, especially during late pregnancy and delivery. There are also small risks of embryopathy from warfarin in early pregnancy but these may have been overstated. Heparin, which has to be given parenterally, does not cross the placental barrier but can still cause bleeding problems in pregnancy. Full intravenous heparin is only suitable for short-term use, and subcutaneous heparin has been introduced for long-term therapy. This regimen is a useful advance but long-term use still has problems of bruising and maternal bone demineralization. The standard treatment of acute thromboembolic events in pregnancy is continuous intravenous heparin followed by either subcutaneous heparin or warfarin, the latter being changed at 36 weeks gestation. In the prophylaxis of thromboembolism, the trend is towards a more selective approach, anticoagulants being given during pregnancy to those at highest risk and during labour and the puerperium to all with a previous history of thromboembolism. Anticoagulants during pregnancy are necessary in patients with artificial heart valves and, because subcutaneous heparin is not sufficient, warfarin should be used until 36 weeks followed by continuous intravenous heparin until delivery. No method of anticoagulation during pregnancy is entirely free of risk and all management policies must be based on an estimate of risk-benefit ratio in individual patients.