Is the association between low birth weight and asthma independent of genetic and shared environmental factors?

Epidemiologic evidence linking birth weight and asthma is inconsistent. The authors examined the association between birth weight and asthma during childhood and adult life in twins. Using prospectively collected data on 21,588 like-sexed Swedish twins of known zygosity born in 1928-1952, they first conducted a cohort study to examine the risk of asthma in relation to birth weight. Next, they conducted nested co-twin control analyses among 643 dizygotic and 365 monozygotic twin pairs discordant for asthma to ascertain whether the association between birth weight and asthma could be confounded by genetic and shared environmental exposures. In the cohort analysis, birth weight of <2,500 g was associated with significantly greater risk of asthma independent of perinatal characteristics and within-twin-pair correlations. In the co-twin control analyses, birth weight of <2,500 g was significantly related to increased risk of asthma among monozygotic twins (relative risk for 2,000 g vs. 2,500 g = 1.58, 95% confidence interval: 1.06, 2.38). A negative association between birth weight and asthma, albeit not statistically significant, was also found among dizygotic twins. In conclusion, there is a negative association between birth weight and asthma in twins that is unlikely to be confounded by genetic or shared environmental factors.