1. The First Clinical Medical College, Lanzhou University, Lanzhou, China
2. Department of Obstetrics and Gynecology, Gansu Provincial Hospital, Lanzhou, China
3. School of Public Health, Lanzhou University, Lanzhou, China
4. Evidence-based Social Sciences Research Center, Lanzhou University, Lanzhou, China
5. Evidence-based Medicine Center, School of Basic Medical Sciences, Lanzhou University, Lanzhou, China
6. Institute of Clinical Research and Evidence Based Medicine, Gansu Provincial Hospital, Lanzhou, China
7. Reproductive Medicine Hospital of the First Hospital of Lanzhou University, Lanzhou, China