[1] Peking Univ, Dept Child Adolescent & Womens Hlth, Sch Publ Hlth, Beijing, Peoples R China
[2] Natl Hlth & Family Planning Commiss PRC, Dept Maternal & Child Hlth, Beijing, Peoples R China
[3] Peking Univ, Dept Epidemiol & Biostat, Sch Publ Hlth, 38 Xueyuan Rd, Beijing 100191, Peoples R China