1. Department of Obstetrics and Gynecology, West China Second University Hospital, Sichuan University, Chengdu, P.R. China
2. Key Laboratory of Birth Defects and Related Diseases of Women and Children, Sichuan University, Ministry of Education, Chengdu, China
3. Department of Obstetrics and Gynecology, Peking University Third Hospital, Beijing, China
4. National Clinical Research Center for Obstetrics and Gynecology, Peking University Third Hospital, Beijing, China
5. National Center for Healthcare Quality Management in Obstetrics, Peking University Third Hospital, Beijing, China