Incidence of Down's syndrome in Sweden during the years 1968-1977.

The incidence of Down's syndrome has been studied among children born in Sweden during the years 1968-1977. The risk for mothers of different ages of bearing such a child did not change during these years. This does not exclude that a change in incidence might have occurred in smaller areas of the country but escaped detection for statistical reasons. A higher than expected number of children with Down's syndrome were born in a few communities, which most likely is a chance event. No correlation could be detected between the incidence of Down's syndrome and a number of socioeconomic variables. The correlation with maternal age was studied in detail. There was a significant excess of males among both the newborn children with Down's syndrome and fetuses with trisomy 21 aborted after prenatal diagnosis. A similar tendency was found among the cases with a chromosome mosaicism but not among those with a translocation. Two hypotheses are put forward to explain the excess of males with trisomy 21.