In women who have prosthetic heart valves, pregnancy is risky for mother and fetus. Heparin has been considered safer for the fetus than warfarin, but may not provide adequate anticoagulation for the mother. We examined prospectively gathered data from 100 pregnancies in 67 women with mechanical valves (age range, 19 to 45 years). A subgroup of 20 patients was compared with a control group of relatives and neighbors who conceived at similar ages. Fetal loss occurred in 44 of the 100 pregnancies, due to the following causes: spontaneous abortion (28), intrauterine fetal death (4), stillbirth (3), neonatal death (1), premature birth (2), Rh incompatibility (2), and maternal death (4). Age, parity, atrial fibrillation, and left ventricular enlargement did not affect the outcome. Tricuspid valve disease that required diuretics was associated with a higher rate of fetal loss (17 out of 23 pregnancies, versus 27 out of 77; p = 0.001), but did not affect the mother Of 66 pregnancies in which the mother was on heparin, 38 (576%) resulted in a healthy baby, compared with 18 out of 34 (52.9%) pregnancies in which the mother was on warfarin (p = NS). All thromboembolic complications occurred with heparin therapy (9 cases; p = 0.02). In the control group, fetal loss was 24 %, due exclusively to spontaneous abortion. Women with mechanical valves have higher rates of fetal loss and maternal complications. In our study, tricuspid valve disease adversely affected fetal outcome, which is a new finding that warrants further study. Warfarin was more effective than heparin in preventing thromboembolism in the mothers, and it did not show a significant impact on the babies.