1. Department of Obstetrics and Gynecology, Dongguan People's Hospital, Dongguan, Guangdong 523000, P.R. China
2. Department of Obstetrics and Gynecology, Midwifery Profession, Shanxi Health Vocational College, Jinzhong, Shanxi 030600, P.R. China
3. The Second Ward of Gynecology, Dalian Obstetrics and Gynecology Hospital Affiliated to Dalian Medical University and Dalian Maternal and Child Health Care Hospital, Dalian, Liaoning 116033, P.R. China