Temporal changes in UK birth order and the prevalence of atopy

Many studies have reported an inverse association between birth order and the risk of respiratory allergic disease. In recent decades, the prevalence of atopy has increased alongside reductions in fertility rates.

To quantitate how much of the increased prevalence of atopy, measured by skin prick test or specific IgE, can be attributed to temporal changes in family size in the United Kingdom.

Through a systematic literature review (MEDLINE, 1965-2009), five studies of UK populations were identified and their data were included in the calculation of a summary odds ratio for the risk of atopy for each birth order. Information on changes in UK family sizes between 1960 and 2001 was obtained from Eurostat. On this basis, expected increases in the prevalence of atopy were calculated by weighting the proportion in each birth order category for 1960 and 2001 by the summary odds ratio for that category and then calculating the relative risk of atopy in 2001 compared with 1960.

The pooled summary odds ratios for atopy were 0.90, 0.69 and 0.69 for those born second, third and fourth (or higher), respectively. The expected relative increase in the prevalence of atopy resulting from a change in family size between 1960 and 2001 was 3%.

Despite the strong associations between birth order and atopy, reductions in family size in the last 40 years account for little of the increase in atopy.