1. Department of Obstetrics and Gynecology, Qilu Hospital, Shandong University, Jinan, Shandong 250012, P.R. China
2. School of Medicine, Shandong�University, Jinan, Shandong 250012, P.R. China
3. Department of Obstetrics and Gynecology, Shanghai General Hospital, School of Medicine, Shanghai Jiao Tong University, Shanghai 201620, P.R. China
4. Department of Obstetrics and Gynecology, Zhenjiang First People's Hospital, Zhenjiang, Jiangsu 212000, P.R. China