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Methods

This paper arose from an extensive literature review undertaken in developing the Canadian Healthy Infant Longitudinal Development (CHILD) study, a multicentre national observational study that is currently in progress. The study, which will eventually recruit 5000 pregnant women, has the aim of determining the environmental, host, genetic and psychosocial risk factors for development of allergy and asthma in children. Although not a systematic review, the examination of epidemiologic risk factors in the development of asthma presented here began in 2004 with a search of MEDLINE, using the Medical Subject Heading (MeSH) terms “asthma,”“longitudinal” and “cohort study.” One of us (PS) reviewed the abstracts of all studies identified in the search, excluding those without at least one objective outcome measure and those in which the primary outcome measure was not asthma. Studies examining the same outcome measure were tabulated but not combined, since most did not consider exactly the same outcome at the same age. We then performed specific searches to fill gaps in the information gathered via the original search, specifically nutrition, sex and gender effects, and novel environmental exposures. The review was updated in July 2008.