Objective To examine whether the season of birth or season of the early phase of gestation is associated with sensitisation to food allergens in children, with special reference to mothers' pollen exposure in spring.

Design A population-based cohort study linking information from a questionnaire survey to allergy tests performed on the target population and regional pollen counts.

Population Children born in 2001–6 who were resident in the province of South Karelia, Finland, at the time of the survey (N=5920).

Main Outcome Measures A positive result in any food allergy test or food-specific immunoglobulin E test (sIgE).

Results The cumulative incidence of a positive food allergy test up to the age of 4 years was highest among children born in October–November (10%) and lowest among those born in June–July (5%), and correspondingly highest among children who were in their 11th gestational week in April–May (11%), the season of high concentrations of birch and alder pollen, and lowest among those reaching that stage in December–January (6%). The amplitude of seasonal variation in any test, estimated as the relative ratio between the peak and trough of the smoothed incidence curve over the year, was 2.03 (95% CI 1.52 to 2.76). The amplitudes of positive sIgE were especially pronounced for milk (3.07; 95% CI 1.81 to 5.50) and egg (3.03; 95% CI 1.86 to 5.18).

Conclusions Children having their early gestational period in the pollen season for broad-leafed trees are more prone to sensitisation to food allergens than other children.