Abstract Background— Pregnant women with a mechanical heart valve (MHV) are at a heightened risk of a thrombotic event, and their absolute need for adequate anticoagulation puts them at considerable risk of bleeding and, with some anticoagulants, fetotoxicity. Methods and Results— Within the prospective, observational, contemporary, worldwide Registry of Pregnancy and Cardiac disease (ROPAC), we describe the pregnancy outcome of 212 patients with an MHV. We compare them with 134 patients with a tissue heart valve and 2620 other patients without a prosthetic valve. Maternal mortality occurred in 1.4% of the patients with an MHV, in 1.5% of patients with a tissue heart valve (P=1.000), and in 0.2% of patients without a prosthetic valve (P=0.025). Mechanical valve thrombosis complicated pregnancy in 10 patients with an MHV (4.7%). In 5 of these patients, the valve thrombosis occurred in the first trimester, and all 5 patients had been switched to some form of heparin. Hemorrhagic events occurred in 23.1% of patients with an MHV, in 5.1% of patients with a tissue heart valve (P<0.001), and in 4.9% of patients without a prosthetic valve (P<0.001). Only 58% of the patients with an MHV had a pregnancy free of serious adverse events compared with 79% of patients with a tissue heart valve (P<0.001) and 78% of patients without a prosthetic valve (P<0.001). Vitamin K antagonist use in the first trimester compared with heparin was associated with a higher rate of miscarriage (28.6% versus 9.2%; P<0.001) and late fetal death (7.1% versus 0.7%; P=0.016). Conclusions— Women with an MHV have only a 58% chance of experiencing an uncomplicated pregnancy with a live birth. The markedly increased mortality and morbidity warrant extensive prepregnancy counseling and centralization of care.