Abstract

OBJECTIVES To document the relation between sibling number and atopic disease, and to assess the contribution of possible confounding factors to the protective effect of siblings in relation to asthma and hay fever.

DESIGN AND SUBJECTS Cross sectional survey by parental questionnaire in Tasmania, Australia, on 6378 children (92% of those eligible) who reached 7 years of age during 1995.

METHODS Exercise challenge lung function testing was conducted on 428 children. Analyses reported were conducted on singleton births only (n = 6158).

RESULTS The prevalences of a history of asthma ever, hay fever, and eczema were 27%, 19%, and 22%, respectively. Asthma and hay fever, but not eczema, were inversely related to sibling number, with evidence of a dose–response trend. The mean age at onset for asthma or wheezy breathing decreased as the number of siblings increased. The inverse association between sibling number and asthma or hay fever persisted after adjustment for several confounders, such as parental smoking or breast feeding, but did not persist after adjustment for household size in 1995.

CONCLUSIONS The protective effect of high sibling number could not be separated from household size at age 7, and it appears to be operating after birth and influences the age at onset of asthma symptoms. Further work to increase knowledge of how the protective effect of the presence of siblings works might have important implications for the understanding of the pathogenesis of asthma.