1. Department of Obstetrics, Tangshan Central Hospital, Tangshan, Hebei, China
2. Department of Obstetrics and Gynaecology, Jingxing County Hospital, Shijiazhuang, Hebei, China
3. Department of Obstetrics, Xingtai Third Hospital, Xingtai, Hebei, China
4. Department of Obstetrics, Fourth People’s Hospital of Langfang, Langfang, Hebei, China
5. Department of Obstetrics and Gynaecology, Zhengding County People’s Hospital, Shijiazhuang, Hebei, China.