Normal pregnancy is accompanied by changes in coagulation that have likely evolved to protect women from the bleeding challenges of miscarriage and childbirth. Consequently, pregnant women are at an increased risk of thrombosis. The most important risk factors are thrombophilia and a history of thrombosis. Most thromboses in pregnancy occur in the left lower extremity, but pelvic vein thromboses are not uncommon. Thrombophilia increases not only the risk of maternal thrombosis but also the risk of poor pregnancy outcome. All pregnant women should be asked about a personal or family history of thrombosis and the details of their obstetrical history. Some women should undergo laboratory testing, particularly those with a personal history of thrombosis or a history of poor pregnancy outcome. The purpose of testing is to help determine which women should receive anticoagulation therapy, which is used not only to treat venous thromboembolism, but also to prevent thromboembolism and reduce the risk of poor pregnancy outcome in women with thrombophilia. Low-molecular-weight heparins are preferred over unfractionated heparin because they have a longer half-life and are presumed to have fewer side effects. Their longer half-life is a disadvantage around the time of delivery when unfractionated heparin, with its shorter half-life, is easier to manage. The risk of thrombosis is higher postpartum than during pregnancy, so anticoagulation therapy is usually continued for at least 6 weeks after delivery.