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

Abstract

Objectives: Infants 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.

Methods: We analyzed 4.1 million infants born at ≥37weeks 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 41weeks of gestation, adjusting for individual characteristics. The main outcomes were mortality due to congenital anomaly, asphyxia, immaturity, infection, sudden infant death, and injury.

Results: Infant mortality decreased progressively from 4.55 per 1000 at 37weeks to 1.62 per 1000 at 41weeks. Early neonatal mortality varied little between 39 and 41weeks, but post neonatal mortality was lowest at 40-41weeks. Relative to 41weeks of gestation, mortality at 39weeks 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).

Conclusion: In Canada, mortality at term is lowest for infants born at 40 or 41weeks of gestation, especially at late and post neonatal ages, and for congenital anomaly and sudden infant death.

Keywords: Gestational age; Infant mortality; Term birth.