1. Department of Maternal and Child Health, School of Public Health, Peking University, National Health Commission Key Laboratory of Reproductive Health, Beijing 046000, China
2. Peking University Health Science Center-Weifang Joint Research Center for Maternal and Child Health, Weifang 261000, China
3. Changzhi Medical College, Changzhi 046000, China
4. Dongcheng Primary and Secondary School Health Care Center, Beijing 236499, China
5. Urumqi Primary and Secondary School Health Care Center, Urumqi 830003, China
6. Mentougou Primary and Secondary School Health Care Center, Beijing 102300, China