1 Department of Obstetrics and Gynecology, Peking Union Medical College Hospital, Peking Union Medical College, Chinese Academy of Medical Sciences, No. 1 Shuaifu Road, Dongcheng District, Beijing, 100730, China.
2 Department of Obstetrics and Gynecology, Peking Union Medical College Hospital, Peking Union Medical College, Chinese Academy of Medical Sciences, No. 1 Shuaifu Road, Dongcheng District, Beijing, 100730, China. zhu_julie@vip.sina.com.