Venous thromboembolism associated with pregnancy is the most common cause of maternal deaths and also is a significant cause of maternal morbidity. The most common genetic factor predisposing to thrombosis is the factor V Leiden (FVL) mutation. Retrospective studies have found a 10% to 25% increase in the risk of venous thromboembolism in pregnant women heterozygous for the FVL mutation. The present prospective, multicenter study was aimed at determining the risk conferred by the FVL mutation on pregnant women with no history of thromboembolism. Another goal was to assess the impact of maternal (or fetal) carriage of the FVL mutation on pregnancy outcomes. Screening of 4885 women who were seen at 13 clinical centers before 14 weeks gestation yielded 134 heterozygous carriers of the FVL mutation (2.7%). Whites were affected substantially more often than blacks or Hispanics.
Of 4 women with pregnancy-related venous thromboembolism, 3 had pulmonary embolism, and one had deep venous thrombosis. None of these events occurred in women carrying the FVL mutation. The risk of adverse pregnancy outcomes did not differ significantly in mutation carriers and noncarriers. The findings in this prospective study indicate strongly that untreated mothers heterozygous for the FVL mutation are at low risk of thromboembolism (and prophylactic anticoagulant therapy is not indicated) providing that there are no evident risk factors for thrombosis.