Season of the first trimester of pregnancy predicts sensitisation to food allergens in childhood: a population-based cohort study from Finland Kaisa Pyrho ¨nen, 1,2,3 Esa La ¨a ¨ra ¨, 4 Liisa Hiltunen, 2,3,5 Minna Kaila, 6 Timo Hugg, 3 Simo Na ¨yha ¨ 3,7 ABSTRACT 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 2001e6 who were resident in the province of South Karelia, Finland, at the time of the survey (N¼5920). 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 OctobereNovember (10%) and lowest among those born in JuneeJuly (5%), and correspondingly highest among children who were in their 11th gestational week in AprileMay (11%), the season of high concentrations of birch and alder pollen, and lowest among those reaching that stage in DecembereJanuary (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. Children born in autumn or winter have a higher occurrence of atopic dermatitis and bronchial asthma and higher concentrations of food-specific immunoglobulin E antibodies than children born in spring or summer. 1e4 The reasons for this are not known but there may be some seasonally varying environmental factors that influence the produc- tion of immunoglobulins. The fetus begins to produce immunoglobulin E antibodies at approxi- mately the 11th gestational week 56 and allergen- specific immunoglobulin E antibodies at the end of the second trimester of pregnancy. 6 An allergic-type immunological response is necessary for the preg- nancy to continue, but in some cases this type of reaction persists throughout the neonatal period. 56 While seasonal environmental factors could inter- fere with this process, the timing of the develop- ment of sensitisation, a prerequisite for clinical immunoglobulin E-mediated allergic disease, has remained a controversial issue. 7e9 In the context of the South Karelian Allergy Research Project (SKARP), 10 (K Pyrhönen et al, manuscript in preparation), an epidemiological study of the occurrence of food allergies and hypersensitivities and risk factors for these, we examined whether the season of birth or the season of the early fetal period was related to the incidence of positive results in food allergy tests during childhood. We also compared the seasonal variation in the incidence with concentrations of pollen grains in the environment and with the relevant climatological data for the study area concerned. METHODS Study population The study population comprised all 5973 children born between 1 April 2001 and 31 March 2006, who were resident in the province of South Karelia in south-east Finland at the time when the ques- tionnaire survey was conducted in 2005e6. The subjects were identified and their demographic details obtained from the Finnish Population Register Centre 11 (K Pyrhönen et al, manuscript in preparation). We estimate that 99% of the children were born in Finland. Data collection Details of the study design and data collection have been reported elsewhere (K Pyrhönen et al, manu- script in preparation). 10 Briefly, we conducted a questionnaire survey in close cooperation with local child health clinics 12 during the period April 2005 to September 2006, when the children were 0e4 years old. Altogether the parents of 3899 chil- dren returned the questionnaire, the participation rate being 66%, varying from 59% to 72% by month of birth. The questionnaires are available at http://kelo.oulu.fi/tutkimus/EKAT/. The results of food allergy tests carried out by order of a clinician from August 2001 to September 2006 were collected from patient and laboratory records at all hospitals, health centres, private clinics and laboratories in the area independently of and concurrently with the questionnaire survey. The test data were intended to cover the whole population, including those who failed to return the questionnaire. The data include the dates and results of the tests and the child’s unique personal identity code. 11 Based on parental reports in the questionnaire we estimated that our test data 1 South Karelia District of Social and Health Services, Lappeenranta, Finland 2 Unit of General Practice, Oulu University Hospital, Finland 3 Institute of Health Sciences, University of Oulu, Finland 4 Department of Mathematical Sciences, University of Oulu, Finland 5 Health Centre of Oulu, Oulu, Finland 6 Paediatric Research Centre, Tampere University Hospital and University of Tampere, Finland 7 Finnish Institute of Occupational Health, Oulu, Finland Correspondence to Dr Kaisa Pyrho ¨nen, South Karelian Institute, Lappeenranta University of Technology, PO Box 20, FIN-53851 Lappeenranta, Finland; kaisa.pyrhonen@oulu.fi Accepted 29 June 2010 Pyrho ¨nen K, La ¨a ¨ra ¨ E, Hiltunen L, et al. J Epidemiol Community Health (2010). doi:10.1136/jech.2009.105411 1 of 8 Research report JECH Online First, published on October 19, 2010 as 10.1136/jech.2009.105411 Copyright Article author (or their employer) 2010. Produced by BMJ Publishing Group Ltd under licence.