1. Zijun Liao, Yubo Zhou, Hongtian Li, and Jianmeng Liu are with the Institute of Reproductive and Child Health/Ministry of Health Key Laboratory of Reproductive Health, and the Department of Epidemiology and Biostatistics, School of Public Health, Peking University, Beijing, China. Cheng Wang is with the Gansu Provincial Maternity and Child-Care Hospital, Lanzhou, Gansu, China. Dunjin Chen is with the Third Affiliated Hospital of Guangzhou Medical University, Guangzhou, Guangdong, China.