1. Institute of Reproductive and Child Health/National Health Commission Key Laboratory of Reproductive Health, Peking University, Beijing 100191, China
2. Department of Epidemiology and Biostatistics, School of Public Health, Peking University, Beijing 100191, China
3. Department of Global Health, School of Public Health, Peking University, Beijing 100191, China
4. Office of Epidemiology, Chinese Center for Disease Control and Prevention, Beijing 102206, China