1. Department of Obstetrics and Gynecology Peking University People’s Hospital Beijing China
2. Department of Obstetrics and Gynecology Chinese PLA General Hospital Beijing China
3. Department of Obstetrics and Gynecology Beijing Hospital Beijing China
4. Department of Obstetrics and Gynecology Peking Union Medical College Hospital Beijing China
5. Department of Obstetrics and Gynecology Beijing Anzhen Hospital Beijing China
6. Department of Obstetrics and Gynecology Beijing Chaoyang Hospital Beijing China
7. Department of Obstetrics and Gynecology Peking University Third Hospital Beijing China
8. Department of Obstetrics and Gynecology Beijing Maternity Hospital Beijing China
9. Department of Obstetrics and Gynecology Beijing Xuanwu Hospital Beijing China
10. Department of Obstetrics and Gynecology Beijing Tongren Hospital Beijing China
11. Department of Obstetrics and Gynecology Beijing Tiantan Hospital Beijing China
12. Department of Obstetrics and Gynecology Beijing Fuxing Hospital Beijing China
13. Department of Obstetrics and Gynecology Beijing Shijitan Hospital Beijing China
14. Department of Statistics Peking University First Hospital Beijing China