A total of 1
602 individuals contributed 8
731 person years under observation between 1989 and 1998. The mean time from birth to censoring or type 1 diabetes was 10.2 (SD=5.0) years, and the mean time under observation after 1 January 1989 was 5.9 (3.3) years. The mean age at diagnosis among the 1824 who developed type 1 diabetes was 8.6 (3.7) years.
We found a weak crude association between maternal age at delivery and incidence of type 1 diabetes. This became somewhat stronger and significant after year of birth and age group were adjusted for (table ). There were no significant crude associations between birth order and incidence of type 1 diabetes (table ). When both maternal age and birth order were included in the model, the rate ratios for maternal age were hardly changed but birth order became weakly negatively associated with type 1 diabetes (test for trend; P=0.06). However, after stratification, an interaction between maternal age at delivery and birth order appeared.
We found no association between maternal age at delivery and incidence of type 1 diabetes among firstborn children, but among second or later born children there was a positive association. The strength of the association increased with birth order (figure). In a regression model with maternal age and birth order entered as continuous variables, their multiplicative interaction was highly significant (P=0.004). Table shows the estimated change in incidence for each five year increase in maternal age from regression models stratified by birth order. For instance, among fourthborn children each five year increase in maternal age was associated with a 43.2% increase (95% confidence interval 6.4% to 92.6%) in incidence of type 1 diabetes.
Estimated associations of maternal age at delivery and birth order with incidence of type 1 diabetes from stratified Poisson regression models with adjustment for age group and calendar period of birth. All values are percentages
Compared with firstborn children, the incidence of type 1 diabetes was lower for second or later children when maternal age was low (figure). For instance, among children born to mothers aged 20-24 years at delivery, each increase in birth order was associated with a 17.9% reduction in the incidence of type 1 diabetes (3.2% to 30.4%). The association was weaker or non-existent when maternal age at delivery was 30 years or more (table ).
There was a weak association between incidence of type 1 diabetes and paternal age at delivery. This was significant after adjustment for calendar period of birth and age group (table ). The same patterns that were found for maternal age were also found for paternal age. However, the strength of the associations and the interaction with birth order were weaker than those for maternal age and the P values were larger (data not shown). After the effects of maternal age and birth order were adjusted for, there was no significant association between paternal age and type 1 diabetes. The effects of maternal age and birth order, however, were essentially unchanged after adjustment for paternal age.
The incidence of type 1 diabetes showed a small non-significant decrease during the study and was slightly higher among boys than girls. We previously found a weak but significant positive association between birth weight and incidence of type 1 diabetes,17
but in this study there was no association between the other potential confounding factors evaluated and incidence of type 1 diabetes. All main results were essentially independent of potential confounders evaluated. The associations were similar for each sex, year of birth, and age group, although the main effect of maternal age was slightly stronger in the earlier birth periods than later periods (P=0.03 for interaction between maternal age and birth period in model with only maternal age and period and in model including age group, birth order, and interaction term between maternal age and birth order).