1. Department of Obstetrics and Gynecology, Peking University First Hospital Ningxia Women and Children’s Hospital (Ningxia Hui Autonomous Region Maternal and Child Health Hospital), Yinchuan, PR. China
2. Department of Obstetrics and Gynecology, Fujian Maternity and Child Health Hospital, College of Clinical Medicine for Obstetrics & Gynecology and Pediatrics, Fujian Medical University, Fuzhou, PR. China
3. Department of Obstetrics and Gynecology, Peking University First Hospital, Beijing, PR. China
4. Department of Obstetrics and Gynecology, Luanzhou People’s Hospital, Luanzhou, PR. China