1. Reproductive Medicine Centre, Weifang People's Hospital, Weifang, Shandong 261041, P.R. China
2. Department of Obstetrics and Gynecology, Zhangqiu District Maternal and Child Health Care Hospital, Jinan, Shandong 250200, P.R. China
3. Department of Nursing, The Third People's Hospital of Qingdao, Qingdao, Shandong 266041, P.R. China