Birth weight and risk of asthma in 3 - 9-year-old twins: Exploring the fetal origins hypothesis

Abstract

Aim: To examine the relationship between birth weight and risk of asthma in a population of twins. Methods: Birth weight of all live twins (8280 pairs) born in Denmark between 1994 and 2000 was linked to information on asthma obtained from parentecompleted 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 weighed on average 122 g (95% CI 85 to 160) 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 100 g) 1.04 (95% CI 1.03 to 1.05), p<0.001. Within twin pairs, the lower birthweight twin had a significantly increased risk of asthma compared with the heavier coetwin (11.3% vs 9.9%), OR 1.30 (95% CI 1.10 to 1.54), p=0.002. The result remained significant after adjusting for sex, birth length and Apgar score, OR 1.31 (95% CI 1.03 to 1.65), p=0.027. The risk tended to be higher in monozygotic coetwins compared with dizygotic coetwins, 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 nonegenetic confounding factors. This finding lends support to the "fetal origins hypothesis" suggesting undisclosed prenatal determinants for the risk of asthma.