1. Department of Obstetrics and Gynecology, Peking University First Hospital, Beijing, PR China,
2. Tongzhou Maternal and Child Health Hospital of Beijing, Beijing, PR China,
3. Department of Obstetrics and Gynecology, Peking University Third Hospital, Beijing, PR China,
4. Department of Obstetrics and Gynecology, General Hospital of Beijing Military Region, Beijing, PR China,
5. Center Hospital of Aviation Industry, Beijing, PR China,
6. Pinggu Maternal and Child Health Hospital of Beijing, Beijing, PR China,
7. Beijing Hospital of Miyun City, Beijing, PR China,
8. Navy General Hospital, Beijing, PR China,
9. Beijing Daxing District Hongxing Hospital, Beijing, PR China,
10. Beijing Chui Yang Liu Hospital, Beijing, PR China,
11. Peking University Shougang Hospital, Beijing, PR China,
12. Combined with Traditional Chinese and Western Medicine Hospital of Beijing City, Beijing, PR China,
13. Beijing No.6 Hospital, Beijing, PR China,
14. Beijing Changping Hospital of Traditional Chinese Medicine, Beijing, PR China, and
15. General Hospital of Jingmei Group, Beijing, PR China