1. Department of Obstetrics and Gynecology, Changhai Hospital, Second Military Medical University, Shanghai 200433, P.R. China
2. Department of Special Clinic, Changhai Hospital, Second Military Medical University, Shanghai 200433, P.R. China
3. Department of Ultrasound, Changhai Hospital, Second Military Medical University, Shanghai 200433, P.R. China
4. Center for Reproductive Medicine, The Second People's Hospital of Changzhou, Changzhou, Jiangsu 213003, P.R. China