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.