Department of Obstetrics and Gynecology Peking Union Medical College Hospital,Peking Union Medical College,Beijing 100730,China;
Department of Obstetrics and Gynecology Peking Union Medical College Hospital,Peking Union Medical College,Beijing 100730,China;
Department of Obstetrics and Gynecology Peking Union Medical College Hospital,Peking Union Medical College,Beijing 100730,China;
Department of Obstetrics and Gynecology Peking Union Medical College Hospital,Peking Union Medical College,Beijing 100730,China;
Department of Obstetrics and Gynecology Peking Union Medical College Hospital,Peking Union Medical College,Beijing 100730,China;
National Research Institute for Family Planning ,Peking Union Medical College Hospital,Peking Union Medical College,Beijing 100730,China;