Gestational age and occurrence of atopy at age 31 - a prospective birth cohort study in Finland

Background It has been suggested that main risk factors for development of allergic diseases operate already during pregnancy and in early childhood. Objective To study the association between gestational age, birth weight, p arity and parental farming with the risk of atopy and asthma in young adult s. Methods 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 d one to three of the most common allergens in Finland and to house dust mite . Data on doctor-diagnosed asthma was obtained from questionnaires. Perinat al data had already been collected during pregnancy. Results The risk of atopy increased linearly with increasing length of preg nancy 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 associ ated 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 in teraction 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. Conclusions The results underline the importance of pregnancy and very earl y childhood in the development of atopy, and suggest that timing of the env ironmental exposure is of importance for the immune system. No association was observed for asthma, which may be due to the multifactorial origins of asthma.