Down syndrome and recent demographic trends in Manitoba.

Two hundred and thirty-three children born in Manitoba with Down syndrome between 1965 and 1974 were ascertained and the maternal ages obtained. Mean maternal age was found to have declined in this period both for all livebirths and to a greater extent for Down syndrome children. Though the proportion of mothers of Down syndrome infants with a maternal age of less than 35 years remained the same, the proportion of mothers aged 35 to 39 years increased and the age specific incidence of Down syndrome became significantly greater for women in this age group. Reduction in the proportion of Down syndrome births to women over 40 years and the increased incidence of Down syndrome in children of women aged 35 to 39 years has important consequences for the planning of amniocentesis programmes.