1. Department of Obstetrics and Gynecology, Beijing Tiantan Hospital, The Fifth Clinical Medical College, Capital Medical University, Beijing, P. R. China
2. Department of Obstetrics and Gynecology, Beijing Tongren Hospital, The Fourth Clinical Medical College, Capital Medical University, Beijing, P. R. China
3. Department of Obstetrics and Gynaecology, IVF Unit, Hammersmith Hospital, Imperial College London, London, UK
4. Department of Hysteroscopic Centre, Fuxing Hospital, Capital Medical University, Beijing, P. R. China