1. Institute of Reproductive and Child Health/National Health Commission Key Laboratory of Reproductive Health, Peking University Health Science Center, Beijing, China
2. Department of Epidemiology and Biostatistics, School of Public Health, Peking University Health Science Center, Beijing, China
3. Department of Obstetrics and Gynecology, Peking University Third Hospital, Beijing, China
4. National Center for Healthcare Quality Management in Obstetrics, Peking University Third Hospital, Beijing, China
5. National Clinical Research Centre for Obstetrics and Gynecology, Peking University Third Hospital, Beijing, China