For this study, eligible infants included those with birth weights of 401-1500g born in the National Institute of Child and Human Development (NICHD) Neonatal Research Network (NRN). Before 48 hours of age, enrolled infants were examined using the New Ballard Score by trained and certified research nurses masked to obstetric estimates of gestational age. A best obstetrical estimate of GA, used for screening of potentially eligible infants, was determined from the last menstrual period, obstetrical measures, and ultrasound findings recorded on the labor and delivery records.