1. Institute of Reproductive and Child Health, Peking University/Key Laboratory of Reproductive Health, National Health Commission of the People’s Republic of China, Beijing 100191, P. R. China
2. Department of Epidemiology and Biostatistics, School of Public Health, Peking University, Beijing 100191, P. R. China
3. Department of Environmental Science and Engineering, Kunming University of Science and Technology, Kunming 650500, P. R. China