1. Perinatal Center for Maternity and Neonate, Yokohama City University Medical Center, 4-57 Urafunecyou, Minami-ku, Yokohama, Kanagawa 232-0024, Japan
2. Department of Biostatistics and Epidemiology, Yokohama City University Graduate School of Medicine and University Medical Center, Yokohama, Japan
3. Department of Obstetrics and Gynecology, Yokohama City University Hospital, Yokohama, Japan