[1] Dartmouth Hlth & Geisel Sch Med Dartmouth, Dept Obstet & Gynecol, One Med Ctr Dr, Lebanon, NH 03756 USA
[2] Univ Arizona, Mel & Enid Zuckerman Coll Publ Hlth, Dept Epidemiol & Biostat, Tucson, AZ USA
[3] Univ Colorado, Sch Med, Dept Pediat, Sect Neonatol, Aurora, CO USA
[4] Oregon Hlth & Sci Univ, Dept Pediat, Portland, OR USA
[5] Univ N Carolina, Carolinas Med Ctr, Dept Obstet & Gynecol, Chapel Hill, NC USA
[6] Boston Univ, Sch Publ Hlth, Dept Biostat, Boston, MA USA
[7] Michigan State Univ, Dept Obstet Gynecol & Reprod Biol, Coll Human Med, E Lansing, MI USA
[8] Boston Univ, Sch Publ Hlth, Dept Community Hlth Sci, Boston, MA USA
[9] Massachusetts Dept Publ Hlth, Div MCH Res & Anal, Boston, MA USA