In rare cases, patients with heterozygous hemophilia show bleeding tendency due to a low factor VIII activity of <20%. Here we describe a case of heterozygous hemophilia that developed during pregnancy. The patient was a 33-year-old Japanese woman whose father and brother both showed elevated nasal bleeding tendencies. However, they had received no medication for this condition. At 26 weeks’ gestation she experienced heavy nasal bleeding. The prothrombin time and platelet count were normal, but the activated partial thromboplastin time was prolonged. Premature rupture of the membrane occurred, and an emergency cesarean section was carried out at 28 weeks’ gestation. A diagnosis of heterozygous hemophilia was made after the cesarean section. Hemophilia crisis and heterozygous hemophilia require special obstetric care, with close liaison with a hematologist. Guidelines for the management of such cases should be available and observed.