Objective:To develop current birth weight norms by gestational age for singleton and twin births in Canada.Methods:Birth weight data were obtained from vital statistics and health department birth registrations for over one million live births in Canada from 1986-1988. Unlikely combinations of birth weight and gestational age were defined within each stratum of multiplicity, gender, gestational age as records with birth weights more than interquartile ranges above the 75th percentile or below 25th percentile. Birth weight percentiles (from first to 99th) by gestational age and sex were calculated for singleton and twin live births.Results:Of the total records, 0.4% were missing data on birth weight or gestational age, and an additional 0.4% were identified as extreme outliers and were excluded from the analysis. Charts of birth weight by gestational age show percentiles 1, 3, 5, 10, 25, 50, 75, 90, 95, 97, and 99 by sex singleton and twin live births.Conclusions:The large data base assembled for this analysis provides current, stable birth weight-gestational age percentiles for classifying newborns from a developed country as small, appropriate, or large for gestational age. Compared with birth weight distributions from the 1970s, these current norms are heavier for full-term infants and the interdecile range for preterm infants is narrower. We recommend that birth weight norms be updated every 5-10 years.(Obstet Gynecol 1993;81:39-48)