[1] Univ Toronto, Womens Coll Hosp, Dept Med, Toronto, ON, Canada
[2] Univ Toronto, Dept Prevent Med, Toronto, ON, Canada
[3] Univ Toronto, Toronto Hosp, Dept Clin Biochem, Toronto, ON, Canada
[4] Univ Toronto, Toronto Hosp, Dept Gynecol, Toronto, ON, Canada
[5] Yale Univ, Dept Epidemiol & Publ Hlth, New Haven, CT 06520 USA