This paper presents an analysis of nine data sets containing data on 4825 Down syndrome cases in which ascertainment and motherly age risk of Down syndrome are estimated jointly using Maximum Likelihood. Estimates of specific occurrence of Down syndrome to maternal age, obtained from data collected prior to the introduction of prenatal screening, are used for the assessment of the effect of environmental factors, for risk assessment and monitoring the effectiveness of screening programs. The literature from which current estimates are obtained is reviewed with focus on methods for dealing with under-reporting. Models including mechanisms for estimating the degree of under-reporting are then fitted to data from the literature. These suggest that the rates currently assumed may under estimate the true rates by about 10%. The primary aim of our analysis is to produce an improved model for live birth Down syndrome prevalence. A secondary aim is to examine the level of under-ascertainment in the different studies.

The results show that, allowing for under-ascertainment, there is a good degree of consistency between the different data sets. A comparison between three and five parameter Constant Plus Exponential model with a three parameter logistic model for maternal age specific risk and suggest a three parameter logistic model for risk assessment were carried out.