The true maternal and fetal risks of heparin therapy during pregnancy are unknown, because most published studies are small and do not report consecutively treated patients. In order to address these issues, we performed a retrospective cohort study of 100 pregnancies in 77 women treated with heparin during pregnancy. In 98 pregnancies heparin therapy was given for the prevention or treatment of venous thromboembolism; in the remaining 2, because of prosthetic heart valves. The rates of prematurity, abortions, stillbirths, neonatal deaths, and congenital abnormalities were similar to those in the normal population. There were two bleeding episodes and no symptomatic thrombotic episodes associated with heparin therapy. We conclude that maternal heparin therapy is safe for the fetus and is associated with an acceptable bleeding rate and a low rate of thrombotic recurrence in the mother.