1. Department of Obstetrics, Shouguang People's Hospital, Weifang, Shandong 262700, P.R. China
2. Department of Gynaecology, The People's Hospital of Qihe County, Qihe, Shandong 251100, P.R. China
3. Department of Laboratory, Jining No. 1 People's Hospital, Jining, Shandong 272000, P.R. China