1. Department of Obstetrics and Gynecology, Sichuan Academy of Medical Sciences and Sichuan Provincial People's Hospital, Chengdu, Sichuan 610072, P.R. China
2. Department of Obstetrics and Gynecology, West China Second University Hospital, Sichuan University, Chengdu, Sichuan 610041, P.R. China
3. Key Laboratory of Birth Defects and Related Diseases of Women and Children (Sichuan University), Ministry of Education, Chengdu, Sichuan 610041, P.R. China
4. Department of Obstetrics and Gynecology, Second Affiliated Hospital, Chongqing Medical University, Chongqing 410010, P.R. China