1. Family Planning Department, Jiangxi Maternal and Child Health hospital, Nanchang, China;
2. Obstetrics & Gynecology Department, Liuzhou Maternal and Child Health-Care Hospital, Liuzhou, China;
3. Obstetrics & Gynecology Department, Peking University Third Hospital, Beijing, China;
4. Family Planning Department, China International Peace Maternity and Child Health Hospital, Shanghai, China;
5. Obstetrics & Gynecology Department, Tongde Hospital of Zhejiang Province, Hangzhou, China;
6. Obstetrics & Gynecology Department, The Second Hospital of Hebei Medical University, Shijiazhuang, China;
7. Obstetrics & Gynecology Department, The Fourth Hospital of Shijiazhuang, Shijiazhuang, China;
8. Obstetrics & Gynecology Department, Chengdu Women’s & Children’s Central Hospital, Chengdu, China;
9. Family Planning Department, Beijing Obstetrics and Gynecology Hospital, Capital Medical University, Beijing, China