Background Pregnancy after mechanical heart valve replacement is highly risky for both mother and child because of the aggravation of maternal heart function and adverse effects of anticoagulation therapy. In Japan, however, the risks and pregnancy outcomes in women with prosthetic mechanical heart valve replacement remain to be elucidated. Methods and Results In the present study 16 pregnancies in 12 women with prosthetic mechanical heart valve replacement were identified between 1983 and 2005. At 6-13 weeks of gestational age, warfarin, an anticoagulant agent, was changed to heparin and administration was continuously adjusted according to the activated partial thromboplastin time level up to the time of delivery. Major maternal complications and pregnancy outcomes were retrospectively investigated. The valve replaced was mitral (n=7), tricuspid (n=7), and aortic (n=2). Eight (50%) of 16 had cesarean live births. One case was delivered at full term, and 7 cases were delivered preterm (26-36 weeks) because of maternal indications. Two babies died in the neonatal period. Therapeutic abortion was performed in 3 cases, 4 cases ended in early miscarriage, and 1 case ended in intrauterine fetal death (30 weeks). Three mothers developed valve (mitral, tricuspid, aortic) thrombosis. There was 1 maternal death from heart failure. Conclusions Pregnancy after mechanical heart valve replacement requires strict control of coagulation. Special attention should be paid to the occurrence of complications during anticoagulation therapy. (Circ J 2007; 71: 211 - 213)