1. Japanese Organization of Prevention of Preterm Delivery (JOPP), Tokyo, Japan
2. Department Obstetrics and Gynecology, Nippon Medical School, Tokyo, Japan
3. Ito Medical Clinic, Tokyo, Japan
4. Department of Obstetrics, Showa University Koto Toyosu Hospital, Tokyo, Japan
5. Department of Obstetrics and Gynecology, University of Toyama, Toyama, Japan
6. Department of Obstetrics and Gynecology, Faculty of Medicine, The University of Tokyo, Tokyo, Japan
7. Department of Obstetrics and Gynecology, Aomori Prefectural Central Hospital, Aomori, Japan
8. Department of Obstetrics and Gynecology, Keio University School of Medicine, Tokyo, Japan
9. Department of Obstetrics and Gynecology, The Jikei University School of Medicine, Tokyo, Japan
10. Department of Obstetrics and Gynecology, Kyorin University, Tokyo, Japan
11. Department of Obstetrics and Gynecology, National Hospital Organization Okayama Medical Center, Okayama, Japan
12. Department of Obstetrics and Gynecology, Jichi Medical University School of Medicine, Tochigi, Japan