1. Department of Child, Adolescent and Women's Health, School of Public Health; Peking University Health Science Center; Beijing; China
2. College of Nursing and Health Sciences; University of Massachusetts-Boston; Boston; Massachusetts; USA
3. Department of Epidemiology and Biostatistics, School of Public Health; Peking University Health Science Center; Beijing; China
4. Laishui County Maternal and Child Health Hospital; Laishui; Hebei; China