A simple function for maternal-age-specific rates of Down syndrome in the 20-to-49-year age range and its biological implications.

The familial increase in the rate of Down syndrome with maternal age can be represented by a simple equation, consisting of the sum of a constant term plus an exponential term that is a first-order function of masternal age: y = a + exp (b + cx), where y is the rate in live births, x is maternal age, and a, b, and c are constants. Unlikely analyses in which two separate equations were derived from different segments of the 20 to 49 maternal age range, this single, simple equation can be applied to the entire range. An unlike previous complex equations that were derived by regression analysis for the entire age range, the component terms can be readily understood as contributions by different etiologic categories. This model fits the data recently available by 1-year intervals about as well as the approach that used separate equations, but it has fewer parameters and requires no ad hoc division of the age range. However, it does not postulate a sharp transition in biological processes around maternal age 30, but, rather, a process continuously accumulating at a constant exponential rate (analogous to that produced by an infectious mechanism), superimposed upon a constant background rate.