Department of Obstetrics and Gynecology, Jichi Medical University, 3311-1 Shimotsuke, Tochigi 329-0498, Japan; Department of Obstetrics and Gynecology, Koga Red Cross Hospital, 1150 Shimoyama, Koga, Ibaraki 306-0014, Japan; Medical Examination Center, Ibaraki Western Medical Center, Chikusei, Japan. Electronic address: