1. Department of Obstetrics and Gynecology, Hokkaido University Graduate School of Medicine, Sapporo, Japan;
2. Department of Obstetrics and Gynecology, JCHO Hokkaido Hospital, Sapporo, Japan;
3. Department of Obstetrics and Gynecology, Sapporo Maternity Women’s Hospital, Sapporo, Japan;
4. Department of Obstetrics and Gynecology, Sapporo Toho Hospital, Sapporo, Japan;
5. Department of Obstetrics and Gynecology, NTT East Japan Sapporo General Hospital, Sapporo, Japan;
6. Department of Obstetrics and Gynecology, Teine Keijinkai Hospital, Sapporo, Japan;
7. Department of Obstetrics and Gynecology, Sapporo City General Hospital, Sapporo, Japan;
8. Department of Obstetrics and Gynecology, School of Medicine, Sapporo Medical University, Sapporo, Japan