It has been suggested that main risk factors for development of allergic diseases operate already during pregnancy and in early childhood. To study the association between gestational age, birth weight, parity and parental farming with the risk of atopy and asthma in young adults. In a prospective birth cohort study, 5192 subjects born in Northern Finland in 1966 were followed up at the age of 31. Skin prick tests were done to three of the most common allergens in Finland and to house dust mite. Data on doctor‐diagnosed asthma was obtained from questionnaires. Perinatal data had already been collected during pregnancy. The risk of atopy increased linearly with increasing length of pregnancy among babies born in the 35th weak of gestation or later. Gestational age equal to, or over 40 weeks compared with less than 36 weeks was associated with an increased risk of atopy (multivariate odds ratio 1.65, 95% CI 1.16, 2.34). The association was stronger among farmers' children ( P for interaction 0.01). High parity and being a farmer's child (multivariate odds ratio 0.50, 95% CI 0.42–0.60) was associated with decreased risk of atopy. In contrast, no associations were observed for doctor‐diagnosed asthma. The results underline the importance of pregnancy and very early childhood in the development of atopy, and suggest that timing of the environmental exposure is of importance for the immune system. No association was observed for asthma, which may be due to the multifactorial origins of asthma.
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