Objective To determine the association between large birth weight and the risk of developing asthma in early childhood.

Methods All single live births (n=687 194) born in Ontario between 1 April 1995 and 31 March 2001 were followed until their sixth birthday. Their birth weight was categorised as low (<2.5 kg), normal (2.5–4.5 kg), large (4.6–6.5 kg) or extremely large (>6.5 kg). Poisson regression analysis was used.

Results Compared with normal-birth-weight infants, large-birth-weight infants (2.3% of total) had a slightly lower risk of developing asthma by age 6 after adjusting for confounders (adjusted RR 0.90, 95% CI 0.86 to 0.93). There was a trend towards increased risk of asthma among extremely large-birth-weight infants (RR 1.21, 95% CI 0.67 to 2.19).

Conclusions Contrary to previous reports, large birth weight was associated with a lower risk for asthma. Instead, a trend towards increased risk of asthma was observed among extremely large-birth-weight infants and interventions to reduce the incidence of extreme large birth weight may help reduce the risk of asthma.