1. Department of Obstetrics (II), People's Hospital of Rizhao, Rizhao, Shandong 276800, P.R. China
2. Intensive Care Unit, People's Hospital of Zhangqiu District, Jinan, Shandong 250200, P.R. China
3. Department of Obstetrics, People's Hospital of Zhangqiu District, Jinan, Shandong 250200, P.R. China