1. Department of Obstetrics, Qingdao Municipal Hospital, Qingdao University, Qingdao, Shandong 266011, P.R. China
2. Department of Gynecology, Qingdao Municipal Hospital, Qingdao University, Qingdao, Shandong 266011, P.R. China
3. Department of Obstetrics and Gynecology, The Affiliated Hospital of Medical College, Qingdao University, Qingdao, Shandong 266003, P.R. China