Inherited thrombophilias are the leading cause of maternal thromboembolism and are associated with an increased risk of certain adverse pregnancy outcomes including second‐ and third‐trimester fetal loss, abruptions, severe intrauterine growth restriction, and early‐onset, severe preeclampsia. Current information suggests that all patients with a history of prior venous thrombotic events and those with these characteristic adverse pregnancy events should be evaluated for thrombophilias. The most common, clinically significant, inherited thrombophilias are heterozygosity for the factor V Leiden and prothrombin G20210A mutations. The autosomal‐dominant deficiencies of protein C and protein S are of comparable thrombogenic potential but are far less common. Homozygosity for the 4G/4G mutation in the type‐1 plasminogen activator inhibitor gene and the thermolabile variant of the methylenetetrahydrofolate reductase gene, the leading cause of hyperhomocysteinemia, although relatively common, confer a low risk of thrombosis. In contrast, autosomal‐dominant antithrombin deficiency and homozygosity or compound heterozygosity (ie, carriers of one copy of each mutant allele) for the factor V and prothrombin mutations are very rare but highly thrombogenic states. Regardless of their antecedent histories, pregnant patients with these highly thrombogenic conditions are at very high risk for both thromboembolism and characteristic adverse pregnancy outcomes, require full therapeutic heparin therapy throughout pregnancy, and need at least 6 weeks of postpartum oral anticoagulation. There is also compelling evidence that patients with the less thrombogenic thrombophilias and a history of venous thrombotic events or characteristic adverse pregnancy outcomes require prophylactic anticoagulant therapy during pregnancy and, in the case of prior thromboembolism, during the puerperium. Antepartum anticoagulation does not appear warranted among patients with less thrombogenic thrombophilias who are without a history of venous thromboembolism, characteristic adverse pregnancy outcomes, or other high risk factors for venous thrombosis.