Down's syndrome and paternal age in Norway.

There is strong evidence for an effect of maternal age on the risk of Down's syndrome. An effect of paternal age has been suspected, but so far neither confirmed nor completely excluded. Large population-based data that allow detailed adjustment for maternal age are needed for a definitive analysis of the paternal age effect. We used data from the Medical Birth Registry of Norway recorded from 1967 to 1998. A total of 1738852 children were included in the analysis. A total of 10.3 per 10000 newborns had Down's syndrome. The data were fitted to logistic regression models with careful control for maternal age, birth calendar year and place of birth. When maternal age was adjusted for using categories of 5-year intervals, residual confounding still resulted in a strong effect of paternal age. However, when the shape of the effect of maternal age was well captured by the model, the estimated effect of paternal age was weak (1.11-fold increased risk per 10 years of paternal age, 95% CI of odds ratio 0.99, 1.22) and not statistically significant.