Corresponding Author

Department of Maternal and Child Health, School of Public Health, Peking University, Beijing, 100191 China

National Health Commission Key Laboratory of Reproductive Health, Beijing, 100191 China

Correspondence

Yuan Wei, Department of Obstetrics and Gynecology, Peking University Third Hospital, 49 North Garden Rd., Haidian District, Beijing, China;

Email: weiyuanbysy@163.com; +86–010–82267852;

Xiaoli Wang, Department of Maternal and Child Health, School of Public Health, Peking University, 38 Xueyuan Rd., Haidian District, Beijing, China;

Email: xlwang@bjmu.edu.cn; +86–010–82801173