1. Department of Obstetrics and Gynecology, The Second Affiliated Hospital of Chongqing Medical University, Chongqing 400010, P.R. China
2. State Key Laboratory of Power Transmission Equipment and System Security and New Technology, Chongqing University, Chongqing 400044, P.R. China
3. Department of Gynecology, Women and Children's Hospital of Guangdong, Guangzhou, Guangdong 511442, P.R. China
4. Equipment Status Evaluation Center, Beijing Electric Power Research Institute, Beijing 100075, P.R. China