The risk of venous thromboembolism increases approximately fivefold during pregnancy. The absolute risk of symptomatic venous thrombosis is 0.5 to 3.0 per 1,000 pregnancies, yet pulmonary embolism is one of the leading causes of pregnancy-associated mortality and morbidity in the United States. The American College of Obstetricians and Gynecologists (ACOG) recently published new clinical management guidelines based on available evidence plus expert opinion for prevention and management of venous thromboembolism in pregnancy.

Changes in a woman's coagulation system account for the hypercoagulable state that occurs during pregnancy. Recently, it has been shown that up to one half of women who experience thrombotic events during pregnancy have an underlying thrombophilia, such as factor V Leiden mutation or another congenital or acquired condition. For a list of risk factors for deep venous thrombosis and other thromboembolic disorders, see Table 1. These women are also at risk for thrombotic events with oral contraceptive use, surgery or immobilization. Women with a history of thromboembolism have a risk of recurrence of up to 12 percent when they become pregnant. The rate of deep venous thrombosis throughout pregnancy is now believed to be comparable to that of postpartum thrombosis; however, pulmonary embolism develops most commonly in the postpartum period.

Use of anticoagulants for thromboprophylaxis is recommended in certain groups of women who are at increased risk of venous thromboembolism during pregnancy. The decision to begin thromboprophylaxis and the regimen chosen must be individualized on the basis of the potential risks and benefits for each woman. In many situations, decisions must be made without evidence from clinical trials. Warfarin and related drugs cross the placenta, and their use is restricted to selected patients late in pregnancy and in whom heparin is contraindicated.

Based on the available evidence and expert opinion, ACOG recommends heparin prophylaxis for pregnant women with any one of the following conditions: (1) artificial heart valves; (2) a history of rheumatic heart disease and atrial fibrillation; (3) antithrombin-III deficiency; (4) antiphospholipid syndrome; (5) a history of rheumatic heart disease with current atrial fibrillation; or (6) a history of anticoagulation therapy. Women homozygous for factor V Leiden mutation and prothrombin G20210A mutation also should receive adjusted-dose heparin, but women with other types of thrombophilias may be at lower risk.

Unfortunately, no definitive data exist to guide dosing in prophylactic heparin regimens. Pregnant women may need double the dosage of those who are not pregnant, and some patients with specific thrombophilias may not respond to heparin. For a list of suggested prophylactic heparin regimens, see Table 2. Low-dose prophylaxis consists of a fixed dose of anticoagulant once or twice daily to prolong the activated partial thromboplastin time (APTT). Adjusted-dose prophylaxis can consist of two to three doses daily plus repeated testing to achieve an APTT of 1.5 to 2.5. In some patients, prophylaxis with heparin may be changed to oral agents such as warfarin during the postpartum period, but management of anticoagulation during labor, delivery and the postpartum period must be individualized.