1. Department of Obstetrics and Gynecology, The Second People's Hospital of Qingyang, Qingyang, Gansu, China
2. Department of Obstetrics and Gynecology, Qingyang Maternal and Child Health Care Hospital, Qingyang, Gansu, China
3. Department of Obstetrics and Gynecology, The First Hospital of Lanzhou University, Lanzhou, China