Impact of low birth weight on early childhood asthma in the United States

Objective: To estimate the independent contribution of birth weight to asthma prevalence among children younger than 4 years in the United States and to compare the magnitude of its effect on asthma between African American and white children. Design: Cross-sectional analysis using the 1988 National Maternal-Infant Health Survey and 1991 Longitudinal Follow-up Survey. Setting: United States. Patients: Eight thousand seventy-one subjects, selected from a randomized, systematic population-based sample and weighted to be nationally representative, who completed both initial and longitudinal follow-up surveys and reported information on asthma diagnosis. Main Outcome Measures: Birth weight and other sociodemographic factors linked to birth outcome were analyzed for independent association with physician-diagnosed asthma by age 3 years. Results: The prevalence of asthma varied by birth weight category: 6.7% in children 2500 g or more at birth, 10.9% in children 1500 to 2499 g at birth, and 21.9% in children less than 1500 g at birth (very low birth weight [VLBW]) (P<.001). Some of the characteristics shown to be independently associated with asthma included: VLBW (odds ratio [OR], 2.9; 95% confidence interval [CI], 2.3-3.6), moderately low birth weight (OR, 1.4; 95% CI, 1.1-1.8), and African American race (OR, 1.9; 95% CI, 1.6-2.4). In stratified analyses, the independent association between VLBW and asthma in white and African American populations was: OR_white, 3.1 (95% CI, 2.2-4.3) and OR_{African American}, 2.5 (95% CI, 2.0-3.3). The prevalence of VLBW, however, was tripled in African American compared with white children (1.8% vs 0.6%). Conclusions: These data confirm findings of other studies that identify a strong independent association between low birth weight and asthma. For this 1988 national birth cohort, an estimated 4000 excess asthma cases were attributable to birth weight less than 2500 g. Although the strength of the independent association between VLBW and asthma was smaller in the African American population, the substantially increased prevalence of VLBW in this community may contribute to the disproportionately increased prevalence of asthma among African American children.