Natural history and associated early life factors of childhood asthma: a population registry-based cohort study in Denmark

Objective Asthma is a common chronic disease that imposes a substantial burden on individuals and society. However, the natural history of childhood asthma in a large population remained to be studied. This study aimed to describe the natural course of childhood asthma and examine the association between early life factors and childhood asthma. Design A population-based cohort study. Setting This study was based on the national registry data in Denmark. Participants All liveborn singletons in Denmark during 1995-1997 were identified and followed them till the end of 2009. Finally, 193 673 children were eligible for our study. Exposures The following characteristics were examined as potential early life factors associated with childhood asthma, including parity, maternal asthma history, maternal smoking during pregnancy, maternal social status, delivery method and gender. Main outcomes and measures Asthma cases were identified on the basis of hospitalisation for asthma and prescriptions for antiasthmatic medications. Asthma remission was defined as no hospitalisation or prescription recorded for 2 years. Cox proportional hazards' regression and logistic regression were used to evaluate the association between early life factors and the occurrence and remission of childhood asthma. Results The cumulative occurrence rate of asthma in children aged 3-14 years was 13.3% and the remission rate was 44.1%. The occurrence rate decreased with age. Being female had a lower risk of asthma (HR: 0.72, 95% CI 0.70 to 0.74) and higher remission rate of asthma (HR: 1.18, 1.13 to 1.22), while maternal asthma was associated with a higher risk of asthma (HR: 2.15, 2.04 to 2.26) and decreased remission rate of asthma (HR: 0.79, 0.73 to 0.85). These patterns remained the same for early onset asthma. Conclusions Female gender had a lower risk of asthma and a better chance of remission, while maternal asthma history had an opposite effect. The early life factors may influence the natural course of childhood asthma.