Li Xiao1, Dong Liu1, Yong Song1, Wei Huang2. 1. Department of Obstetrics and Gynecology, West China Second University Hospital of Sichuan University, Chengdu, Sichuan 610041, China. 2. Department of Obstetrics and Gynecology, West China Second University Hospital of Sichuan University, Chengdu, Sichuan 610041, China. Email: weihuang64@163.com.