Mathematical function for maternal age in Down's syndrome.

The purpose of this study is to fit a mathematic function to the observed risk figures for Down's syndrome at various maternal ages. Data from the New York State and Swedish studies were used. It was found that the estimated risks fitted the exponential relationship very well, with a correlation coefficient of over 0.94. An even better correlation was found (over 0.97) for each 5-yr period in the New York State study. The exponential relationship was given by: I = aebx, where 1/I is the estimated risk; x, the maternal age in years; and a and b, the constants. In both studies there was considerable variation in the values of a and b with maternal age as well as an excellent correlation between ln a and b for the 5-yr periods. This function may be used to test various hypotheses for the effect of maternal age in Down's syndrome.