Cause-specific infant mortality in a population-based Swedish study of term and post-term births: the contribution of gestational age and birth weight

OBJECTIVE To investigate infant mortality and causes of infant death in relation to gestational age (GA) and birth weight for GA in non-malformed term and post-term infants. DESIGN Observational, retrospective nationwide cohort study. SETTING Sweden 1983-2006. PARTICIPANTS 2 152 738 singleton non-malformed infants born at 37 gestational weeks or later. MAIN OUTCOME MEASURES Infant, neonatal and postneonatal mortality and causes of infant death. RESULTS Infant mortality rate was 0.12% (n=2687). Compared with infants born at 40 weeks, risk of infant mortality was increased among early term infants (37 weeks, adjusted OR 1.70, 95% CI 1.43 to 2.02). Compared with infants with normal birth weight for GA, very small for gestational age (SGA; <3rd percentile) infants faced a doubled risk of infant mortality (adjusted OR 2.13, 95% CI 1.80 to 2.53), and corresponding risk was also increased among moderately SGA infants (3rd to <10th percentile; adjusted OR 1.46, 95% CI 1.26 to 1.68). Sudden infant death syndrome (SIDS) was the most common cause of death, accounting for 39% of all infant mortality. Compared with birth at 40 weeks, birth at 37 weeks was associated with increased risks of death by infections, cardiovascular disorders, SIDS and malignant neoplasms. Very and moderately SGA were associated with increased risks of death by neonatal respiratory disorders, infections, cardiovascular disorders, SIDS and neuromuscular disorders. High birth weight for GA was associated with increased risks of death by asphyxia and malignant neoplasms. CONCLUSION Early term birth and very to moderately low birth weight for GA are independent risk factors for infant mortality among non-malformed term infants.