Infant mortality at term in Canada: Impact of week of gestation

•Week-to-week variation in risk of mortality for infants born at term is poorly understood.•Infants born at least 40 weeks, and particularly at 41 weeks, have the lowest risk of mortality in Canada.•Risks at 40 weeks are lower for congenital anomaly, asphyxia, infection, sudden infant death, and injury.

ObjectivesInfants born at term have low mortality, but risk may vary from week to week. We determined the risk of infant mortality at term by gestational week and cause.MethodsWe analyzed 4.1 million infants born at ≥ 37 weeks of gestation in Canada from 1991 to 2010, followed for mortality the first year of life. We estimated hazard ratios (HR) and 95% CIs for early, late and post neonatal mortality from 37 through 41 weeks of gestation, adjusting for individual characteristics. The main outcomes were mortality due to congenital anomaly, asphyxia, immaturity, infection, sudden infant death, and injury.ResultsInfant mortality decreased progressively from 4.55 per 1000 at 37 weeks to 1.62 per 1000 at 41 weeks. Early neonatal mortality varied little between 39 and 41 weeks, but post neonatal mortality was lowest at 40-41 weeks. Relative to 41 weeks of gestation, mortality at 39 weeks was higher for congenital anomaly (HR 1.30, 95% CI 1.05-1.60) and sudden infant death (HR 1.58, 95% CI 1.18-2.11).ConclusionIn Canada, mortality at term is lowest for infants born at 40 or 41 weeks of gestation, especially at late and post neonatal ages, and for congenital anomaly and sudden infant death.