Maternal age patterns and the incidence of Down's syndrome.

The incidence of Down's syndrome in three counties in southwestern Ontario from 1960 to 1974 was monitored in this retrospective descriptive study. Of 229, 754 live births including 292 Down's syndrome births, there was an incidence of 1.27/1,000 live births. The age-adjusted incidence for 288 cases where mother's age was known was 1.25/1,000. Data from Statistics Canada demonstrated a decrease in proportions of births to women over 35 years and a decrease in mean maternal age. The anticipated decrease in incidence of Down's syndrome because of these factors was not found; a real biological increase to women under 35 years cannot be ruled out.