1 Department of Obstetrics and Gynecology, The Medical University of Ningxia (Ms. Wei and Mr. Ma).
2 Department of Obstetrics and Gynecology, The General Hospital of Ningxia Medical University (Dr. Ha and Mss. Li and Guo), Yinchuan, Ningxia, China.
3 Department of Obstetrics and Gynecology, The General Hospital of Ningxia Medical University (Dr. Ha and Mss. Li and Guo), Yinchuan, Ningxia, China. Electronic address: [email protected].