1. Department of Obstetrics and Gynaecology, Beijing Shijitan Hospital, Capital Medical University, Beijing 100038, P.R. China
2. Cell Collection and Research Centre, National Institutes for Food and Drug Control, Beijing 100050, P.R. China
3. Department of Gynaecology, Beijing Maternal and Child Health Hospital of Haidian District, Beijing 100080, P.R. China