Anticoagulant therapy is indicated during pregnancy for the prevention and treatment of venous thromobo-embolism and for the prevention and treatment of systemic embolism in patients with prosthetic heart valves and other valvular heart disorders. Several questions concerning anticoagulant therapy remain unanswered. Although it is highly likely that heparin therapy is safe for the fetus, the true risk of clinically important maternal osteoporosis after long-term heprain therapy is unknown. Oral anticoagulants are fetopathic, but the true risks of the warfarin embryopathy and central nervous system abnormalities are unknown. There is some evidence that warfarin embryopathy occurs only when oral anticoagulants are administered between the 6th and 12th weeks of gestation and that oral anticoagulants may not be fetopathic when administrated in the first 6 weeks of gestation. The fetal risks also are probably lower after the 12th week of gestation than between the 6th and 12th weeks, but warfarin is probably still fetopathic when administered during the 2nd and 3rd trimesters. Oral anticoagulant therapy should be avoided in the weeks before delivery because of the risk of serious perinatal bleeding caused by the trauma of delivery to the severely anticoagulated fetus.