[1] Univ Calgary, Dept Paediat, Cumming Sch Med, Alberta Childrens Hosp,Res Inst, Calgary, AB T2N 1N4, Canada
[2] Columbia Univ, Med Ctr, Dept Obstet & Gynecol, Div Maternal Fetal Med, New York, NY USA
[3] Univ Washington, Dept Obstet & Gynecol, Seattle, WA 98195 USA
[4] Washington Univ, Sch Med, Dept Obstet & Gynecol, St Louis, MO 63110 USA
[5] Washington Univ, Sch Med, Pathol & Immunol, St Louis, MO USA
[6] Sahlgrens Univ Hosp, Inst Hlth Women & Children, Dept Obstet & Gynecol, Gothenburg, Sweden
[7] Norwegian Inst Publ Hlth, Div Epidemiol, Dept Genes & Environm, Oslo, Norway