Although most patients with uterine myomas are asymptomatic during pregnancy, profound enlargement of a pedunculated myoma has serious consequences. In the present case, gradual enlargement of a pedunculated myoma was observed throughout a woman's pregnancy, worsening after delivery and causing hypovolemic shock, without evidence of external or intra-abdominal hemorrhage. Laparotomy revealed a severely edematous myoma containing a large amount of blood. It is speculated that partial occlusion had obstructed venous return, but not arterial blood flow, which caused rapid enlargement of the myoma.