Maternal and paternal age at delivery, birth order, and risk of childhood onset type 1 diabetes: population based cohort study.

OBJECTIVE To estimate the associations of maternal and paternal age at delivery and of birth order with the risk of childhood onset type 1 diabetes. DESIGN Cohort study by record linkage of the medical birth registry and the national childhood diabetes registry in Norway. SETTING Norway. SUBJECTS All live births in Norway between 1974 and 1998 (1.4 million people) were followed for a maximum of 15 years, contributing 8.2 million person years of observation during 1989-98. 1824 cases of type 1 diabetes diagnosed between 1989 and 1998 were identified. MAIN OUTCOME MEASURES Incidence of type 1 diabetes. RESULTS There was no association between maternal age at delivery and type 1 diabetes among firstborn children, but among fourth born children there was a 43.2% increase in incidence of diabetes for each five year increase in maternal age (95% confidence interval 6.4% to 92.6%). Each increase in birth order was associated with a 17.9% reduction in incidence (3.2% to 30.4%) when maternal age was 20-24 years, but the association was weaker when maternal age was 30 years or more. Paternal age was not associated with type 1 diabetes after maternal age was adjusted for. CONCLUSIONS Intrauterine factors and early life environment may influence the risk of type 1 diabetes. The relation of maternal age and birth order to risk of type 1 diabetes is complex.