1. Department of Pediatrics, Lanzhou University Second Hospital, Lanzhou, Gansu, China
2. Institute of Epidemiology and Health Statistics, School of Public Health, Lanzhou University, Lanzhou, Gansu, China
3. Gansu maternal and Child Health Hospital, Lanzhou, Gansu, China
4. The Second School of Clinical Medicine, Lanzhou University, Lanzhou, Gansu, China