The relationship between birth weight and childhood asthma: a population-based cohort study.

BACKGROUND Because obesity promotes inflammation and imposes mechanical constraints to the airways, a high birth weight may be a risk factor for asthma in childhood. However, to our knowledge, few studies have examined this potential relationship. OBJECTIVE To determine the relationship between high birth weight and risk of emergency visits for asthma during childhood. DESIGN Population-based cohort study. SETTING Alberta, Canada. PARTICIPANTS All neonates born at term (> or =37 weeks) between April 1, 1985, and March 31, 1988, in Alberta (N = 83,595). We divided the cohort into birth-weight categories: low (<2.5 kg), normal (2.5-4.5 kg), or high (>4.5 kg). The cohort was observed prospectively for 10 years.Main Outcome Measure Comparison of risk of emergency visits for asthma over 10 years across the birth-weight categories. RESULTS Neonates born with a high birth weight had a significantly increased risk of emergency visits for asthma during childhood compared with neonates born with a normal birth weight (relative risk [RR], 1.16; 95% confidence interval [CI], 1.04-1.29). The relationship between birth weight and emergency visits for asthma beyond a birth weight of 4.5 kg was linear, such that every increment of 0.10 kg in birth weight was associated with an additional 10% (95% CI, 2%-19%) increase in the risk of emergency visits for asthma. Other factors associated with an elevated risk for emergency asthma visits during childhood included male sex (RR, 1.26; 95% CI, 1.22-1.30), aboriginal status (RR, 1.20; 95% CI, 1.11-1.29), and low-income status (RR, 1.11; 95% CI, 1.06-1.16). CONCLUSIONS A high, but not low, birth weight is a risk factor for increased emergency visits during childhood. The risk increases linearly beyond a birth weight of 4.5 kg.