1. Department of Gynecology, General Hospital of Ningxia Medical University, Yinchuan, Ningxia 750001, P.R. China
2. Department of Gynecology, Hospital of Cardiovascular and Cerebrovascular Diseases, General Hospital of Ningxia Medical University, Yinchuan, Ningxia 750002, P.R. China
3. Reproductive Medicine Center, Affiliated Maternity and Child Health Care Hospital of Nantong University, Nantong, Jiangsu 226000, P.R. China