1. Institute of Reproductive and Child Health; Peking University/Key Laboratory of Reproductive Health; National Health and Family Planning Commission of the People's Republic of China; Beijing China
2. Department of Epidemiology and Biostatistics; School of Public Health; Peking University
3. Maternal and Child Health Hospital; Tongzhou District of Beijing
4. Maternal and Child Health Hospital; Shunyi District of Beijing
5. Maternal and Child Health Hospital; Fangshan District of Beijing