1. Department of Obstetrics; Hokkaido University Hospital; Sapporo Japan
2. Department of Obstetrics and Gynecology; Jichi Medical University Hospital; Shimotsuke Japan
3. Department of Obstetrics and Gynecology; University of Tsukuba Hospital; Tsukuba Japan
4. Perinatal Center for Maternity and Neonate; Yokohama City University Medical Center; Yokohama Japan
5. Department of Obstetrics and Gynecology; Hamamatsu University Hospital; Hamamatsu Japan