1. Department of Obstetrics; Hokkaido University Graduate School of Medicine; Sapporo Japan
2. Department of Obstetrics and Gynecology; Hakodate Central General Hospital; Hakodate Japan
3. Department of Obstetrics and Gynecology; Sapporo City General Hospital; Sapporo Japan
4. Department of Obstetrics and Gynecology; Japan Community Health Care Organization Hokkaido Hospital; Sapporo Japan
5. Department of Obstetrics and Gynecology; Asahikawa Medical University; Asahikawa Japan