1. Department of Obstetrics and Gynecology, Women's Hospital of Nanjing Medical University, Nanjing Maternity and Child Health Care Hospital, Nanjing 210004, P.R. China
2. Centre for Diseases Prevention and Control of Eastern Theater, Nanjing 210018, P.R. China
3. Department of Infectious Disease, Jiangsu Province Hospital and Nanjing Medical University First Affiliated Hospital, Nanjing 210029, P.R. China
4. Nanjing Normal University, Nanjing 210023, P.R. China
5. Changzhou Maternal and Child Health Care Hospital, Changzhou Medical Center, Nanjing Medical University, Changzhou 213004, P.R. China