1. Department of Obstetrics and Gynecology, Second Affiliated Hospital Harbin, Harbin Medical University, Harbin, Heilongjiang, China (mainland)
2. Department of Obstetrics and Gynecology, Harbin Red Cross Central Hospital, Harbin, Heilongjiang, China (mainland)
3. Department of Obstetrics and Gynecology, Chaoyang Central Hospital, Chaoyang, Heilongjiang, China (mainland)
4. Department of Obstetrics and Gynecology, Harbin Medical University, Harbin, Heilongjiang, China (mainland)