Down syndrome in live births by single year maternal age interval in a Swedish study: comparison with results from a New York State study.

Abstract

Observed and estimated rates of Down syndrome births in Sweden in 1968-70 by single year maternal age interval were analyzed. The 438 Down syndrome births observed spanned maternal ages 16-48 years. A gradual increase in the rates of Down syndrome with maternal age was observed up to age 30-31 years; thereafter a steeper increase occurred. Rates derived from an iterative weighted least squares regression analysis assuming different 1st-degree exponential functions for ages 20-31 and 33 and over were in good agreement with observed rates. Rates derived from a 3rd-degree exponential equation fitted over the entire age range also generally agreed with the observed rate but predicted much higher rates at ages 47-49 years that obtained using the 1st-degree exponential regression equation. The rates predicted using either regression approach were similar to those drived in a independent sttudy in New York State except at the uppermost extreme of maternal age. It is stressed however that the rates observed in this study at any particular age are subject to a certain sampling variation. Although there is some evidence of a separate paternal age effect on the risk of a Down syndrome birth this is assumed to be significant in only a small number of cases because of the generally strong correlation between parental ages.