1. Department of Obstetrics, The Second People's Hospital of Liaocheng, Liaocheng, Shandong 252600, P.R. China
2. Department of Obstetrics, Zhangqiu Maternity and Child Care Hospital, Jinan, Shandong 250200, P.R. China
3. Department of Obstetrics, Qingdao Central Hospital, Qingdao, Shandong 266042, P.R. China