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Background: Severity of airflow obstruction in patients with asthma can be influenced by features beyond lung function impairment. The aim of this study was to assess the severity of obstruction according to the 2022 European Respiratory Society/American Thoracic Society standards, and its associations with perinatal factors, as well as potential interactions with background and clinical factors.

Methods: The study population consisted of 44,394 patients with asthma, aged 7–49 years, with at least one measurement of forced expiratory volume in 1 s (FEV1) and forced vital capacity (FVC) recorded in the Swedish National Airway Register in 2014–2022, who were included in the Medical Birth Register between 1973 and 2015 with data on gestational age (GA) and birthweight. Airflow obstruction was defined as pre-bronchodilator FEV1/FVC < lower limit of normal and categorized as mild, moderate, or severe based on FEV1z-score. Normal FEV1served as the reference category.

Results: Among 7873 (17.7 %) patients with airflow obstruction, the prevalence of severe obstruction pre-bronchodilator was 6.1 %. Patients born extremely-to-very preterm or small for GA (SGA) had increased relative risk ratios for severe obstruction pre-bronchodilator (RRRadj2.34, 95 % CI 1.24–4.41, and 2.03, 1.38–2.98) compared with those born at term and appropriate for GA, respectively, with obstruction and normal FEV1. For GA and birthweight, there were non-significant interactions with background or clinical factors.

Conclusions: Severe obstruction was prevalent in children and adults with asthma. Patients born extremely-to-very preterm or SGA exhibited a significant association with severe obstruction pre-bronchodilator, regardless of background and clinical factors.