1. Department of Gynaecology, Binzhou City Center Hospital, Binzhou, Shandong 251700, P.R. China
2. Department of Obstetrics, Binzhou City Center Hospital, Binzhou, Shandong 251700, P.R. China
3. Department of Gynaecology, The Second People's Hospital of Liaocheng, Linqing, Shandong 252600, P.R. China