Background and objective Genetic factors alone cannot explain the risk of developing coeliac disease (CD). Children born in summer months are likely to be weaned and introduced to gluten during winter when viral infections are more frequent. Earlier studies on birth season and CD are limited in sample size and results are contradictory.

Method Case–control study. We used biopsy reports from all 28 Swedish pathology departments to identify individuals with CD, defined as small intestinal villous atrophy (n=29 096). The government agency Statistics Sweden then identified 144 522 controls matched for gender, age, calendar year and county. Through conditional logistic regression we examined the association between summer birth (March–August) and later CD diagnosis (outcome measure).

Results Some 54.10% of individuals with CD versus 52.75% of controls were born in the summer months. Summer birth was hence associated with a small increased risk of later CD (OR 1.06; 95% CI 1.03 to 1.08; p<0.0001). Stratifying CD patients according to age at diagnosis, we found the highest OR in those diagnosed before age 2 years (OR 1.17; 95% CI 1.10 to 1.26), while summer birth was not associated with a CD diagnosis in later childhood (age 2–18 years: OR 1.02; 95% CI 0.97 to 1.08), but had a marginal effect on the risk of CD in adulthood (age ≥18 years: OR 1.04; 95% CI 1.01 to 1.07).

Conclusions In this study, summer birth was associated with an increased risk of later CD, but the excess risk was small, and general infectious disease exposure early in life is unlikely to be a major cause of CD.