Underlying maternal and pregnancy-related conditions account for a substantial proportion of neonatal morbidity in late preterm infants.

AIM: We studied the impact of maternal and pregnancy-related conditions and the effect of gestational age itself, on the health of infants born late preterm.

METHODS: Singletons born in gestational weeks 34 + 0 to 41 + 6 in 1995-2013 in the southern region of Sweden were identified from a perinatal register. We found 14 030 infants born late preterm and 294 814 born at term. A hierarchical system was developed to examine the impact of pregnancy complications. The outcomes studied were as follows: neonatal death, central nervous system (CNS) or respiratory disease, infection, neonatal admission and respiratory support. Odds ratios (OR) and 95% confidence intervals (95% CI) were obtained using logistic regression analyses.

RESULTS: Late preterm infants were at increased risk for all outcomes compared to term infants, with adjusted ORs from 13.1 (95% CI: 12.7-13.6) for neonatal admission to 2.3 (95% CI: 1.8-2.9) for infections. Late preterm birth after preterm prelabour rupture of membranes was associated with an overall lower risk compared to late preterm births due to other causes. Exposure to antepartum haemorrhage or maternal diabetes increased the risk for CNS and respiratory morbidity.

CONCLUSION: Morbidity decreased in late preterm infants with increasing gestational age. Underlying conditions accounted for a substantial part of the morbidity.