Pregnancy and thrombotic risk.

Pregnancy increases the risk of thrombosis three-fold to four-fold. The main reason for the increased risk is hypercoagulability, which has likely evolved to protect women against the bleeding challenges associated with miscarriage and childbirth. The hypercoagulability of pregnancy is present as early as the first trimester and so is the increased risk of thrombosis. Factors that further increase the risk of thromboembolism in pregnancy include a history of thrombosis, inherited and acquired thrombophilia, certain medical conditions, and complications of pregnancy and childbirth. Candidates for anticoagulation are women with a current thrombosis, a history of thrombosis, thrombophilia, and a history of poor pregnancy outcome, or risk factors for postpartum thrombosis. For fetal reasons, the preferred agents for anticoagulation in pregnancy are heparins. There are no large trials of anticoagulation in pregnancy and recommendations are based on case series and the opinion of experts. Nonetheless, anticoagulation is believed to improve the outcome of pregnancy for women at risk.