[1] Johns Hopkins Univ, Sch Med, Dept Gynecol & Obstet, Baltimore, MD 21218 USA
[2] Friends Res Inst, Baltimore, MD USA
[3] Univ Rochester, Sch Med & Dent, Dept Family Med, Rochester, NY USA
[4] Johns Hopkins Univ, Sch Med, Biostat Epidemiol & Data Management, Baltimore, MD USA
[5] Johns Hopkins Univ, Sch Med, Dept Pediat, Baltimore, MD USA
[6] Johns Hopkins Bloomberg Sch Publ Hlth, Dept Hlth Behav & Soc, Baltimore, MD USA