The aim of this retrospective study was to compare the different anticoagulation regimens used in pregnant women with prosthetic heart valves. We reviewed 196 pregnancies in 110 women from 1974 to 2000. The patients were divided into two groups: group 1 (142 pregnancies) had warfarin throughout pregnancy; and in group 2 (54 pregnancies), warfarin was replaced by subcutaneous heparin during the first trimester and last two weeks of pregnancy. There were no maternal complications in 129 pregnancies in group 1 and 44 in group 2. There were significantly fewer normal births in group 1 (56; 39.4%) compared to group 2 (39; 72.2%). Group 1 had a significantly higher rate of spontaneous abortion (46.5% vs 14.8%), but group 2 had a higher rate of valve thrombosis. In group 1, women with a warfarin requirement < 5 mg had a lower rate of spontaneous abortion. Warfarin is an effective anticoagulant in pregnant women with mechanical valves but it results in significant fetal loss when the dose is > 5 mg. Heparin is a less effective anticoagulant resulting in more maternal complications, but it is more protective of the fetus.