1. Department of Obstetrics, Chengdu Women and Children Center Hospital, Chengdu, Sichuan 610000, P.R. China
2. Department of Gynaecology and Obstetrics, First People's Hospital of Jinan, Jinan, Shandong 250011, P.R. China
3. Department of Orthopedics, The Second Affiliated Hospital of Fujian Medical University, Quanzhou, Fujian 362000, P.R. China