[1] Northwestern Univ, Dept Pediat, Feinberg Sch Med, Chicago, IL 60611 USA
[2] Wayne State Univ, Dept Pediat, Detroit, MI 48202 USA
[3] Univ Alabama Birmingham, Div Neonatol, Birmingham, AL USA
[4] RTI Int, Stat & Epidemiol Unit, Res Triangle Pk, NC USA
[5] Brown Univ, Dept Pediat, Providence, RI 02912 USA
[6] Indiana Univ Sch Med, Dept Pediat, Indianapolis, IN 46202 USA
[7] RTI Int, Stat & Epidemiol Unit, Rockville, MD USA
[8] Eunice Kennedy Shriver Natl Inst Child Hlth & Hum, NIH, Bethesda, MD USA