1. Department of Gynecology and Obstetrics, The Second Affiliated Hospital of Nantong University/The First People’s Hospital of Nantong, Nantong, China
2. Department of Epidemiology and Medical Statistics, School of Public Health, Nantong University, Nantong, China
3. Department of Epidemiology, Fielding School of Public Health, University of California, Los Angeles, CA, USA