Ruixi Zhan1, Hongwu Wen1, Xuelian Gao1, Ling Yin2. 1. Department of Obstetrics and Gynecology, Peking University First Hospital, Beijing 100034, China. 2. Department of Obstetrics and Gynecology, Peking University First Hospital, Beijing 100034, China. Email: yinling65@sina.com.