[1] Cent S Univ, Sch Publ Hlth, Changsha, Hunan, Peoples R China
[2] Univ Ottawa, Fac Med, Dept Obstet & Gynecol, OMNI Res Grp, Ottawa, ON K1H 8L6, Canada
[3] Ottawa Hlth Res Inst, Clin Epidemiol Program, Ottawa, ON K1H 8L6, Canada
[4] Univ Ottawa, Fac Med, Dept Epidemiol & Community Med, Ottawa, ON K1H 8L6, Canada
[5] Univ Med & Dent New Jersey, Sch Publ Hlth, Div Epidemiol, Piscataway, NJ 08854 USA