[1] Univ Penn, Pediat, Wynnewood, PA USA
[2] Univ Penn, Div Maternal Fetal Med, Philadelphia, PA 19104 USA
[3] Univ Maryland, Dept Microbiol & Immunol, Baltimore, MD 21201 USA
[4] Univ Penn, Dept Obstet & Gynecol, Maternal & Child Hlth Res Ctr, Perelman Sch Med, Philadelphia, PA 19104 USA