1. Department of Obstetrics and Gynecology, Peking University First Hospital, Beijing, China
2. Department of Obstetrics and Gynecology, The First Affiliated Hospital of Zhengzhou University, Zhengzhou, China
3. Department of Obstetrics and Gynecology, The Second Affiliated Hospital of Hebei Medical University, Hebei Province, China
4. Department of Obstetrics, Xiangya Hospital Central South University, Changsha, Hunan, China
5. Department of Obstetrics and Gynecology, Nanjing Drum Tower Hospital, The Affiliated Hospital of Nanjing University Medical School, Nanjing, Jiangsu Province, China