Rationale: The relation between birth weight and respiratory symptoms and asthma in children remains unclear. Previous studies focused on a relation at separate ages. A longitudinal analysis may lead to a better understanding.
Objectives: To estimate the effect of birth weight on the development and course of respiratory symptoms and asthma in the first 7 years of life.
Methods: In a prospective birth cohort study, 3,628 children with a gestational age 37 weeks or more were monitored for 7 years. Parental questionnaires were used to assess respiratory health yearly. Associations of birth weight with respiratory symptoms (wheezing, coughing, respiratory infections) and doctor's diagnosis of asthma were assessed in a repeated-event analysis.
Measurements and main results: Lower birth weight was associated with more respiratory symptoms (odds ratio [OR] per kg decrease in birth weight, 1.21; 95% confidence interval [CI], 1.09-1.34). The effect of birth weight increased from age 1 to 5, but decreased thereafter and was no longer significant at the age of 7. The effect of birth weight on respiratory symptoms was significantly greater among children exposed to tobacco smoke in their home than among nonexposed children (OR at 5 yr: 1.21 [95% CI, 1.02-1.44] and 1.52 [95% CI, 1.23-1.87], respectively). Birth weight and a doctor's diagnosis of asthma were not related (OR, 1.06; 95% CI, 0.82-1.37).
Conclusions: A lower birth weight in children born at term is associated with a transiently increased risk of respiratory symptoms. This effect is enhanced by environmental tobacco smoke exposure.