1. School of Public Health, Lanzhou University, 222 South Tianshui Road, Lanzhou 730000, China
2. Department of Developmental and Behavioral Pediatrics, The Children’s Hospital, Zhejiang University School of Medicine, National Clinical Research Centre for Child Health, Hangzhou 310051, China
3. Gansu Provincial Maternity and Child-Care Hospital, Lanzhou 730050, China
4. Department of Public Health, Robbins College of Health and Human Sciences, Baylor University, Waco, TX 76706, USA