Cent S Univ, Sch Publ Hlth, Changsha 410008, Hunan, Peoples R China.
Nanhua Univ, Dept Epidemiol, Sch Publ Hlth, Hengyang, Hunan, Peoples R China.
Univ Ottawa, OMNI Res Grp, Dept Obstet & Gynecol, Fac Med, Ottawa, ON, Canada.
Shanxi Med Univ, Dept Hlth Stat, Taiyuan, Shanxi, Peoples R China.
[Tan, Hongzhuan] Cent S Univ, Sch Publ Hlth, Xiangya Rd 110, Changsha 410008, Hunan, Peoples R China.