Jie Yan1, Huixia Yang2. 1. Department of Obstetrics and Gynecology, Peking University First Hospital, Xi'anmen Street No. 1, Beijing 100034, China. 2. Department of Obstetrics and Gynecology, Peking University First Hospital, Xi'anmen Street No. 1, Beijing 100034, China. Electronic address: yanghuixia@bjmu.edu.cn.