1 Department of Obstetrics and Gynecology, Peking Union Medical College Hospital, Chinese Academy of Medical Sciences and Peking Union Medical College, No. 1 Shuaifuyuan, Dongcheng District, Beijing, 100730, China.
2 National Clinical Research Center for Obstetric and Gynecologic Diseases, Beijing, China.
3 Department of Obstetrics and Gynecology, Peking Union Medical College Hospital, Chinese Academy of Medical Sciences and Peking Union Medical College, No. 1 Shuaifuyuan, Dongcheng District, Beijing, 100730, China. lengjenny@vip.sina.com.
4 National Clinical Research Center for Obstetric and Gynecologic Diseases, Beijing, China. lengjenny@vip.sina.com.