Anticoagulant therapy is sometimes required during pregnancy either for the prevention of thromboembolic disease, for patients already on long-term antithrombotic treatment (for valvular prostheses) or for the prevention of complications of risk factors such as hereditary or acquired thrombophilia. Pregnancy is in itself a hyper-coagulable condition and the risk of thromboembolic complications is raised. Anticoagulation is a risk to the mother and to the foetus, and the management (heparin or vitamin K antagonists, respective doses) must be adapted to the underlying pathology and the stage of pregnancy. Mechanical valve prostheses are the most difficult problem and different strategies are proposed. The use of low molecular weight heparin may improve the outcome of these patients, but further trials are necessary.