Rates of Down syndrome at livebirth by one-year maternal age intervals in studies with apparent close to complete ascertainment in populations of European origin: a proposed revised rate schedule for use in genetic and prenatal screening.

Precision and accuracy in determining rates of Down syndrome at livebirth are indispensible to algorithms which determine eligibility for prenatal cytogenetic diagnostic services. We derived Down syndrome rates by single year of maternal age which we propose as a revised rate schedule for background risk. Data on European-origin populations were obtained from 5 sources judged most likely to have complete ascertainment of cases. A "constant plus exponent" regression model and variants extending the analysis to higher powers of maternal age were applied to several ranges of maternal age. Confidence intervals about the rates were calculated. This analysis results in rates significantly higher than those in widespread use though the confidence intervals show a need for caution in assuming precision. Sources of variation in rates are also considered.