Place and time aspects of the occurrence of Down's syndrome.

Prevalence rates at birth for Down's syndrome were compared for Massachusetts communities of different population densities. Overall, there was a small positive association between the rate of Down's syndrome and population density, entirely attributable to a strong association for the children of older mothers. An analysis for space and time clustering of births of 2469 children born in Massachusetts with Down's syndrome revealed no aggregation of cases, except for a slight seasonal peak in the summer.