1. Department of Maternal and Child Health, School of Public Health, Peking University, Beijing 100191, China
2. Peking University Health Science Center-Weifang Joint Research Center for Maternal and Child Health, Weifang 261000, China
3. Department of Preventive Medicine, Changzhi Medical College, Changzhi 046000, China
4. Urumuqi Primary and Secondary School Health Care Center, Urumuqi 830003, China
5. Dongcheng Primary and Secondary School Health Care Center, Beijing 100010, China
6. Mentougou Primary and Secondary School Health Care Center, Beijing 102300, China