A 30-year-old woman was admitted to our hospital because of thrombocythemia during pregnancy. Her leukocyte count was 10,000/microliters, Hb was 11.7 g/dl, and platelet count 181.9 x 10(4)/microliter. Bone marrow aspirate showed an increase in megakaryocytes (255/microliters). Both Ph1 chromosome and bcr rearrangement were negative. She was diagnosed as having essential thrombocythemia (ET) with pregnancy, and was treated with aspirin (150 mg/day). Her pregnancy was uneventful, but she was readmitted because of overterm pregnancy. A caesarean section was performed, and a healthy male infant weighing 3,672 g was delivered, with a platelet count of 25.5 x 10(4)/microliter. However, the uterine was atonic, and atonic hemorrhage occurred. Supravaginal hysterectomy was performed. Subsequently, intrabdominal gross hemorrhage occurred, but the bleeding was halved by platelet transfusion. Microscopic examination showed uterine infarction. We suggest that platelet count should be reduced by means of plateletpheresis or interferon-alpha throughout pregnancy with ET.