[1] Johns Hopkins Bloomberg Sch Publ Hlth, Dept Populat & Family Hlth Sci, Baltimore, MD 21205 USA
[2] Johns Hopkins Med Inst, Div Maternal Fetal Med, Baltimore, MD 21205 USA
[3] Johns Hopkins Sch Med, Dept Pediat, Baltimore, MD USA
[4] Johns Hopkins Bloomberg Sch Publ Hlth, Dept Biostat, Baltimore, MD USA
[5] Johns Hopkins Sch Med, Dept Gynecol & Obstet, Baltimore, MD USA