Aim: To examine the relationship between birth weight and risk of asthma in a population of twins.
Methods: Birth weight on all live twins (8,280 pairs) born in Denmark between 1994 and 2000 was linked to information on asthma obtained from parent-completed questionnaires at age 3-9 years. Conditional logistic regression was used to calculate the risk of asthma.
Results: Subjects with a history of asthma at age 3-9 years weighted on average 122g, 95% CI (85-160g) less at birth than subjects who had not developed asthma, p<0.001. There was a linear increase in asthma risk with decreasing birth weight, OR(per 100g)=1.04 (1.03-1.05), p<0.001. Within twin pairs, the lower-birth weight twin had a significantly increased risk of asthma compared with the heavier co-twin (11.3 vs. 9.9%), OR=1.30 (1.10-1.54), p=0.002. The result remained significant after adjusting for sex, birth length and Apgar score, OR=1.31 (1.03-1.65), p=0.027. The risk tended to be higher in monozygotic co-twins compared with dizygotic co-twins, especially for high birth weight differences.
Conclusions: Low birth weight is a risk factor for asthma independently of gestational age, sex, birth length and Apgar score, but this may be due, in part, to residual non-genetic confounding factors. This finding lends support to the ‘fetal origins hypothesis’ suggesting undisclosed prenatal determinants for the risk of asthma.
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