Objective To examine the association between birth weight for gestational age and asthma in childhood and adolescence while controlling for potential confounders and considering smoking as an effect modifier.
Methods A retrospective cohort of all singleton term births in Nova Scotia, Canada, between 1989 and 1993 was identified in the provincial perinatal database and followed through 2014 by linking with administrative health data. The outcome, asthma, was defined as having one hospitalisation or two physician visits with an International Classification of Diseases code for asthma in a 2-year period. Birth weight was categorised as small (SGA), large (LGA) or appropriate (AGA) for gestational age. Multivariable-adjusted Cox proportional hazards models were used to examine the association between the birth weight for gestational age and asthma and to test for effect modification by maternal smoking in pregnancy.
Results Of the 40 724 cohort children, 10.5% and 11.7% were born SGA and LGA, respectively, and the risk of developing asthma to age 18 years was 30.2%. The adjusted HRs for SGA and LGA (relative to AGA) and asthma were 1.07 (95% CI 1.02 to 1.14) and 0.96 (95% CI 0.91 to 1.02), respectively. Relative to AGA children born to non-smoking mothers, SGA children were not at increased risk of asthma (HR 1.02), whereas both AGA and SGA children born to smoking mothers were at significantly increased risk (HR 1.14 and 1.29, respectively).
Conclusions Our findings suggest that SGA in term infants is not associated with asthma in childhood in the absence of smoking in pregnancy.
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Contributors JHC contributed to the design of the study, wrote the initial manuscript draft, reviewed and revised the manuscript and approved the final manuscript as submitted. CGW contributed to the design of the study, reviewed and revised the manuscript and approved the final manuscript as submitted. LL carried out the analysis, reviewed and revised the manuscript and approved the final manuscript as submitted. SK conceptualised and designed the study, carried out the analysis, reviewed and revised the manuscript and approved the final manuscript as submitted.
Funding This work was funded by an operating grant from the Canadian Institutes of Health Research (CIHR) awarded to SK (FRN 134531). JHC held a Dalhousie Faculty of Medicine Summer Studentship.
Disclaimer Although this research is based on data obtained from the Nova Scotia Department of Health and Wellness, the observations and opinions expressed are those of the authors and do not represent those of either Health Data Nova Scotia or the Department of Health and Wellness.
Competing interests None declared.
Patient consent Not required.
Ethics approval IWK Health Centre Research Ethics Board (File No 1015756), Halifax, NS, Canada.
Provenance and peer review Not commissioned; externally peer reviewed.
Data sharing statement The data used in this study are available from the Reproductive Care Program of Nova Scotia and Health Data Nova Scotia, Canada. As these data constitute personal health information, they can only be accessed from within Nova Scotia as per provincial privacy laws. Researchers must submit a data access application to the database custodians and upon approval sign a data sharing agreement.