1. Department of Reproductive Medicine, Affiliated Hospital of Weifang Medical University, Weifang, Shandong 261000, P.R. China
2. Department of Obstetrics, Zhucheng People's Hospital, Zhucheng, Shandong 262200, P.R. China
3. Department of Obstetrics and Gynecology, Liaocheng Second People's Hospital, Liaocheng, Shandong 252000, P.R. China
4. Department of Obstetrics and Gynecology, Dezhou People's Hospital, Dezhou, Shandong 253000, P.R. China