1. Department of Obstetrics and Gynecology, Qilu Hospital, Cheeloo College of Medicine, Shandong University, Jinan, Shandong 250012, P.R. China
2. Department of Obstetrics and Gynecology, Jining Medical University Affiliated Tengzhou Central People's Hospital, Tengzhou, Shandong 277599, P.R. China
3. Department of Obstetrics and Gynecology, Tai'an City Central Hospital, Tai'an, Shandong 271000, P.R. China