There is extreme variation in the definition of low plasma glucose levels in newborn infants in the first postnatal days, ranging from < 30 to < or = 60 mg/dL. The goal of the present study was to define low thresholds (< or = 5th percentile) of plasma glucose concentrations in full-term normal newborns during the first 72 hours of life. Population meta-analysis was performed on published studies of neonatal hypoglycemia ascertained by MedLine search. One-way analysis of variance was computed across the studies for each of the following four postnatal time periods: 1 to 2 (physiological nadir), 3 to 23, 24 to 47, and 48 to 72 hours. The estimated < or = 5th percentiles of neonatal hypoglycemia during 1 to 2, 3 to 23, 24 to 47, and 48 to 72 hours after birth were < or = 28, < or = 40, < or = 41, and < or = 48 mg/dL, respectively. Based on this statistical definition, we recommend that low thresholds of plasma glucose levels of 28, 40, and 48 mg/dL be adopted in full-term normal newborns at 1 to 2, 3 to 47, and 48 to 72 hours of life, respectively.