Objectives To estimate the national rate of early-term live births in Brazil and to evaluate the effect of birth at 37 and 38 weeks’ gestation, as compared with 39 and 40 weeks’ gestation on infant outcomes according to precursors of birth and the existence of maternal/fetal medical conditions. Design National perinatal population-based cohort study. Setting 266 maternity services located in the five Brazilian macroregions. Participants 18 652 singleton live newborns from 37 0/7 to 40 6/7 weeks of gestation. Main outcome measures Resuscitation in delivery room, oxygen therapy, transient tachypnoea, admission to neonatal intensive care unit (NICU), hypoglycaemia, use of antibiotics, phototherapy, phototherapy after hospital discharge, neonatal death and breastfeeding. Results Early terms accounted for 35% (95% CI 33.4% to 36.7%) of all live births. Among provider-initiated births in women without medical conditions, infants of 37 and 38 weeks’ gestation had higher odds of oxygen therapy (adjusted OR (AOR) 2.93, 95% CI 1.72 to 4.98 and AOR 1.9295% CI 1.18 to 3.13), along with admission to NICU (AOR 2.01, 95% CI 1.18 to 3.41 and AOR 1.56, 95% CI 1.02 to 2.60), neonatal death (AOR 14.40, 95% CI 1.94 to 106.69 and AOR 13.76,95% CI 2.84 to 66.75), hypoglycaemia in the first 48hours of life (AOR 7.86, 95% CI 1.95 to 31.71 and AOR 5.76, 95% CI 1.63 to 20.32), transient tachypnoea (AOR 2.98, 95% CI 1.57 to 5.65 and AOR 2.12, 95% CI 1.00 to 4.48) and the need for phototherapy within the first 72hours of life (AOR 3.59, 95% CI 1.95 to 6.60 and AOR 2.29, 95% CI 1.49 to 3.53), yet lower odds of breastfeeding up to 1hour after birth (AOR 0.67, 95% CI 0.53 to 0.86 and AOR 0.87, 95% CI 0.76 to 0.99) and exclusive breastfeeding during hospital stay (AOR 0.68, 95% CI 0.51 to 0.89 and AOR 0.84, 95% CI 0.71 to 0.99). Conclusion Birth at 37 and 38 weeks’ gestation increased the risk of most adverse infant outcomes analysed, especially among provider-initiated births and should be avoided before 39 weeks’ gestation in healthy pregnancies.