LAUSR

Background Preterm birth is associated with pulmonary complications early in life; however, long-term risks of asthma into adulthood are unclear. Objective To determine asthma risks from childhood into adulthood associated with gestational age at birth in a large population-based cohort. Methods A national cohort study was conducted of all 4 079 878 singletons born in Sweden during 1973–2013, followed up for asthma identified from primary care, specialty outpatient and inpatient diagnoses in nationwide registries through 2018 (up to 46 years). Cox regression was used to adjust for potential confounders, and cosibling analyses assessed the influence of unmeasured shared familial (genetic and/or environmental) factors. Results In 91.9 million person-years of follow-up, 607 760 (14.9%) persons were diagnosed with asthma. Preterm birth was associated with increased risk of asthma at ages <10 years (adjusted HR 1.73; 95% CI 1.70 to 1.75), 10–17 years (1.29; 1.27 to 1.32) and 18–46 years (1.19; 1.17 to 1.22). Across all ages, adjusted HRs further stratified were 3.01 (95% CI 2.88 to 3.15) for extremely preterm (22–27 weeks), 1.76 (1.72 to 1.79) for very or moderately preterm (28–33 weeks), 1.31 (1.29 to 1.32) for late preterm (34–36 weeks) and 1.13 (1.12 to 1.14) for early term (37–38 weeks), compared with full-term (39–41 weeks) birth. These findings were not explained by shared familial factors. Asthma risks were elevated after spontaneous or medically indicated preterm birth and with or without perinatal respiratory complications. Conclusions In this large national cohort, preterm and early term birth were associated with increased risks of asthma from childhood into midadulthood. Persons born prematurely need long-term follow-up into adulthood for timely detection and treatment of asthma.