1. Yuetan Community Health Service Center of Fu Xing Hospital, Capital Medical University, Beijing, China
2. Obstetrics and Gynaecology of Fu Xing Hospital, Capital Medical University, Beijing, China
3. Obstetrics and Gynaecology of Daxing District People’s Hospital, Beijing, China
4. Yayuncun Community Health Service Center, Chaoyang District, Beijing, China
5. Dongba Community Health Service Center, Chaoyang District, Beijing, China
6. Xinjiekou Community Health Service Center, Xicheng District, Beijing, China