1. Department of Obstetrics, Qilu Hospital of Shandong University, Jinan, Shandong 250010, P.R. China
2. Department of Obstetrics, The Lanshan District Maternal and Child Care Service Centre of Rizhao, Rizhao, Shandong 276812, P.R. China
3. Department Obstetrics, Taian City Central Hospital, Taian, Shandong 271000, P.R. China