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

Abstract

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.

Article summary

Article focus

• Term infants (born at 37 gestational weeks or more) contribute with 30% to all neonatal mortality. Infants born at 37 and 38 weeks have higher rates of infant mortality than infants born at 40 weeks. Little is known about the interplay between GA and birth weight for GA and its effect on infant mortality.

Key messages

• This study adds detailed analyses of the relationships between GA and birth weight for GA and risks of neonatal and postneonatal mortality and causes of infant death. We conclude that induced deliveries before 39 weeks gestation should be avoided when possible and that extra caution should be taken in term pregnancies with suspected severe or moderate intrauterine growth restriction.

Strengths and limitations of this study

• The main strengths of this study are related to sample size and to the large number of predefined risk factors and confounders. Limitations were that some malformations may not have been detected, causing a theoretical selection bias, and that time trends may have influenced the outcome.