The factor V Leiden mutation is the most common genetic risk factor for deep vein thrombosis: it is present in about 5% of the white population. The risk of deep vein thrombosis among women who use oral contraceptives is greatly increased by the presence of the mutation. The same seems to be true of the risk of postpartum thrombosis. Several authors have called for all women to be screened before prescription of oral contraceptives and during pregnancy. Such a policy might deny effective contraception to a substantial number of women while preventing only a small number of deaths due to pulmonary emboli. Moreover, in pregnancy the ensuing use of oral anticoagulation prophylaxis might carry a penalty of fatal bleeding that is equal to or exceeds the risk of death due to postpartum thrombosis. It might pay, however, to take a personal and family history of deep vein thrombosis when prescribing oral contraceptives or at a first antenatal visit to detect women from families with a tendency to multiple thrombosis.