1. Department of Obstetrics and Gynecology, Xi’an Fourth Hospital, Xi’an, Shaanxi 710004, China
2. Department of Obstetrics, Affiliated Hospital of Yan’an University, Yan’an, Shaanxi 716000, China
3. Department of Gynecology, Shaanxi Nuclear Industry 215 Hospital, Xianyang, Shaanxi 712000, China