Background: Previous studies have suggested that, in addition to genetic liability and environment in early childhood, intrauterine life also influences the risk for asthma beyond childhood. Low birth weight, prematurity, young maternal age, and maternal smoking have all shown an association with asthma. The effect of perinatal factors on the risk for asthma in relation to familial and social risk factors was studied in a nationwide population-based sample of adolescent twins. In addition to a distribution of birth characteristics among twins which differs from that of singletons, data on twins enable a distinction to be made between genetic and environmental sources of variation.
Methods: Questionnaires were sent to five consecutive birth cohorts of Finnish 16 year old twins born in 1975-9 and to their parents (3065 families). The outcome measure was life time prevalence of doctor-diagnosed asthma in these adolescents. The association between asthma and potential risk factors was assessed by multiple logistic regression and discordant twin pair analysis.
Results: Risk for asthma increased with increasing ponderal index (p for trend <0.01) and decreasing maternal age (p for trend <0.05). Among the 25% of twins with the highest ponderal index, the odds ratio for asthma was 1.82 (95% confidence interval 1.18 to 2.79) compared with those in the lowest 25%. Neither birth weight, gestational age, nor Apgar score was associated with asthma. When perinatal risk factors were combined with familial and social risk factors, ponderal index, maternal smoking, parental asthma, and sibship size were all significant independent determinants of asthma in these adolescents.
Conclusions: The risk for asthma in adolescent twins increases with increasing ponderal index when adjusted for familial and social factors.