Background The Factor V Leiden and prothrombin-gene mutations are independent risk factors for venous thrombosis; it is debated whether a mutation in the gene encoding methylenetetrahydrofolate reductase, an enzyme involved in homocysteine metabolism, also increases the risk of venous thrombosis. Whether any of these mutations is associated with an increased risk of late fetal death is not known. Methods We studied 67 women with a first episode of unexplained late fetal loss (fetal death after 20 weeks or more of gestation) and 232 women who had had one or more normal pregnancies and no late fetal losses. All the women were tested for the presence of three gene mutations. Women with other thrombophilic conditions were excluded from the study. Results Eleven of the 67 women with late fetal loss (16 percent) and 13 of the 232 control women (6 percent) had either the factor V or the prothrombin mutation. The relative risks of late fetal loss in carriers of the factor V and prothrombin mutations were 3.2 (95 percent confidence interval, 1.0 to 10.9) and 3.3 (95 percent confidence interval, 1.1 to 10.3), respectively. Thirteen percent of the women whose fetuses died and 20 percent of the control women were homozygous for the mutation in the methylenetetrahy-drofolate reductase gene (relative risk, 0.8; 95 percent confidence interval, 0.5 to 1.2). Conclusions Both the factor V and the prothrombin mutations are associated with an approximate tripling of the risk of late fetal loss. (C) 2000, Massachusetts Medical Society.
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