Review of published cases of pregnancies in which coumarin derivatives or heparin were administered demonstrates that use of either class of anticoagulant carries substantial risks. Of 418 reported pregnancies in which coumarin derivatives were used, one-sixth resulted in abnormal liveborn infants, one-sixth in abortion or stillbirth and, at most, two-thirds in apparently normal infants. In addition to the expected hemorrhagic complications, fetal effects of coumarin derivative administration include a specific embryopathy and central nervous system abnormalities. All available cases (including unpublished ones) of warfarin embryopathy and central nervous system abnormalities following gestational exposure to coumarin derivatives are reviewed, various complications are tabulated, critical periods of teratogenesis are discussed and possible mechanisms proposed. The use of heparin during gestation does not result in a significantly better outcome of pregnancy. In 135 published cases, the infants in one-eighth were stillborn, in one-fifth premature (a third of whom died) and, again at most, in two-thirds apparently normal. Because of the substantial risks of both clases of anticoagulants, and the inherent risks of pregnancy complicated by the indications for anticoagulation, prevention of pregnancy is usually indicated. If pregnancy occurs, a relatively normal outcome can be anticipated in about two-thirds of the pregnancies regardless of the anticoagulant used. Heparin does not appear to be a clearly superior alternative to coumarin derivatives.