Prothrombin thrombophilia is characterized by venous thromboembolism (VTE) manifest most commonly in adults as deep-vein thrombosis (DVT) in the legs or pulmonary embolism. The clinical expression of prothrombin thrombophilia is variable; many individuals heterozygous or homozygous for the 20210G>A F2 variant never develop thrombosis, and while most heterozygotes who develop thrombotic complications remain asymptomatic until adulthood, some have recurrent thromboembolism before age 30 years. The relative risk for DVT in adults heterozygous for the 20210G>A variant is two- to fivefold increased; in children, the relative risk for thrombosis is three- to fourfold increased. Heterozygosity for 20210G>A has at most a modest effect on recurrence risk after a first episode. Although prothrombin thrombophilia may increase the risk for pregnancy loss, its association with preeclampsia and other complications of pregnancy such as intrauterine growth restriction and placental abruption remains controversial. Factors that predispose to thrombosis in prothrombin thrombophilia include: the number of 20210G>A alleles; presence of coexisting genetic abnormalities including factor V Leiden; and acquired thrombophilic disorders (e.g., antiphospholipid antibodies). Circumstantial risk factors for thrombosis include pregnancy and oral contraceptive use. Some evidence suggests that the risk for VTE in 20210G>A heterozygotes increases after air travel. [from GeneReviews]