Y Tang1, Y Liu2, H Yu3, F Shen4, M Zhao5, Q Chen6. 1. Department of Family Planning, The Hospital of Obstetrics & Gynaecology, Fudan University, Shanghai, China. 2. School of Medicine, Nanjing Medical University, Nanjing, China. 3. Department of Ultrasound, The Affiliated Wuxi Maternity and Child Health Care Hospital of Nanjing Medical University, Wuxi, China. 4. Department of Obstetrics and Gynaecology, Suzhou Ninth People's Hospital, Suzhou, China. 5. Department of Gynaecology, The Affiliated Wuxi Maternity and Child Health Care Hospital of Nanjing Medical University, Wuxi, China. 6. Department of Obstetrics & Gynaecology, The University of Auckland, Auckland, New Zealand.