Birth weight and childhood onset type 1 diabetes: population based cohort study.

Objective: To assess the associations between birth weight or gestational age and risk of type 1 diabetes.

Design: Population based cohort study by record linkage of the medical birth registry and the National Childhood Diabetes Registry.

Setting: Two national registries in Norway.

Participants: All live births in Norway between 1974 and 1998 (1 382 602 individuals) contributed a maximum of 15 years of observation, a total of 8 184 994 person years of observation in the period 1989 to 1998. 1824 children with type 1 diabetes were diagnosed between 1989 and 1998.

Main Outcome Measures: Estimates of rate ratios with 95% confidence intervals for type 1 diabetes from Poisson regression analyses.

Results: The incidence rate of type 1 diabetes increased almost linearly with birth weight. The rate ratio for children with birth weights 4500 g or more compared with those with birth weights less than 2000 g was 2.21 (95% confidence interval 1.24 to 3.94), test for trend P=0.0001. There was no significant association between gestational age and type 1 diabetes. The results persisted after adjustment for maternal diabetes and other potential confounders.

Conclusion: There is a relatively weak but significant association between birth weight and increased risk of type 1 diabetes consistent over a wide range of birth weights.