[1] Natl Inst Environm Hlth Sci, Epidemiol Branch, POB 12233, Durham, NC 27709 USA
[2] Ctr Fertil & Hlth, Oslo, Norway
[3] Oregon Hlth & Sci Univ, Portland State Univ, Sch Publ Hlth, Portland, OR USA
[4] Oregon Hlth & Sci Univ, Dept Obstet & Gynecol, Portland, OR USA
[5] Univ British Columbia, Dept Obstet & Gynaecol, Vancouver, BC, Canada
[6] McGill Univ, Dept Obstet & Gynecol, Montreal, PQ, Canada
[7] McGill Univ, Dept Epidemiol Biostat & Occupat Hlth, Montreal, PQ, Canada