| Summary: | In this study, we sought to ascertain a relationship between gestational age at birth and infectious morbidity of the offspring via population-based cohort analysis comparing the long-term incidence of infectious morbidity in infants born preterm and stratified by extremity of prematurity (extreme preterm birth: 24 + 0–27 + 6, very preterm birth: 28 + 0–31 + 6, moderate to late preterm birth: 32 + 0−36 + 6 weeks of gestation, and term deliveries). Infectious morbidity included hospitalizations involving a predefined set of International Classification of Diseases 9 (ICD9) codes, as recorded in hospital records. A Kaplan–Meier survival curve compared cumulative incidence of infectious-related morbidity. A Cox proportional hazards model controlled for confounders and time to event. The study included 220,594 patients: 125 (0.1%) extreme preterm births, 784 (0.4%) very preterm births, 13,323 (6.0%) moderate to late preterm births, and 206,362 term deliveries. Offspring born preterm had significantly more infection-related hospitalizations (18.4%, 19.8%, 14.9%, and 11.0% for the aforementioned stratification, respectively, <i>p</i> < 0.001). Multivariate analysis found being born very or late to moderate preterm was independently associated with long-term infectious morbidity (adjusted hazard ratio (aHR) 1.5, 95% confidence interval (CI) 1.27–1.77 and aHR 1.23, 95% CI 1.17–1.3, respectively, <i>p</i> < 0.001). A comparable risk of long-term infectious morbidity was found in the two groups of premature births prior to 32 weeks gestation. In our population, a cutoff from 32 weeks and below demarks a significant increase in the risk of long-term infectious morbidity of the offspring.
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