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Prenatal or perinatal environmental exposures have been postulated to change the risk of inflammatory bowel disease (IBD) later in life.1 One manifestation of this, perhaps because of early and seasonal infectious exposure, might be an association between the month of birth and risk of developing IBD later in life.2–4 Two recent publications, one from the UK,3 and the other from Denmark4 have examined cohorts of Crohn’s patients for such seasonality of birth. The findings of these two studies as well as their interpretations differ markedly. The British study was of adult patients diagnosed between 1972 and 1989 by four regional IBD registers in which a weak association was found between Crohn’s disease (CD) and birth in the first half of the year. The second (Danish) study examined national computerised hospital discharge records of children and young people aged less than 20 years coded between 1977 and 1992 for CD. This second study found an apparently stronger association with month of birth with the peak risk being for those born during August with a “sine wave” variation in risk between those born in different months of the year. The authors suggest that the differences compared with the British study might be explained by differences in age mix and nationality of the subjects, and further suggested that their findings …