aDepartment of Epidemiology and Biostatistics, Ministry of Education Key Laboratory of Environment and Health, School of Public Health, Tongji Medical College, Huazhong University of Science and Technology, Wuhan

bKey Laboratory of Birth Defects and Related Diseases of Women and Children (Sichuan University), Ministry of Education, West China Second University Hospital, Sichuan University

cShuangliu Institute of Women's and Children's Health, Shuangliu Maternal and Child Health Hospital, Chengdu, China

dDepartment of Epidemiology and Population Health, Albert Einstein College of Medicine, Bronx, NY, USA

Correspondence to An Pan, PhD, Department of Epidemiology and Biostatistics, School of Public Health, Tongji Medical College, Huazhong University of Science and Technology, 13 Hangkong Rd, Wuhan 430030, China. Tel: +86 27 83627309; e-mail: [email protected]