The safest and most practical method of administering long-term anticoagulants in pregnancy is uncertain because treatment of the mother with vitamin K antagonists may be complicated by hemorrhage in the fetus. The effects on the fetus of giving coumadin in pregnancy was evaluated in rabbits. When coumadin was given from early pregnancy until term, all of the fetuses were stillborn with widespread hemorrhages. However, the fetuses were born alive and without hemorrhage when (1) coumadin was stopped 4-5 days before delivery, at which time the level of coagulation factors had almost returned to normal and (2) when delivery was performed by cesarean section at a time when the fetal coagulation defect was severe. It is suggested that the risk of fetal hemorrhage is high only when fetuses with a severe coagulation defect are exposed to the trauma of delivery.