[1] CUNY, Sch Publ Hlth, Dept Epidemiol & Biostat, 55 West,125th St, New York, NY 10027 USA
[2] Univ Calgary, Dept Obstet & Gynaecol, Calgary, AB, Canada
[3] Univ British Columbia, Sch Populat & Publ Hlth, Dept Family Practice, Vancouver, BC, Canada
[4] CUNY, Sch Publ Hlth, Dept Community Hlth & Social Sci, New York, NY 10021 USA