Calculating correct Down's syndrome risks

Women having both first trimester nuchal translucency and second trimester serum screening tests are likely to receive two different Down's syndrome r isks. Neither will be correct, and we describe how to calculate a valid com bined risk. This uses the reported serum-based risk and a likelihood ratio derived from the nuchal translucency report. Tables, figures and examples a re provided to aid the calculation of the likelihood ratio from either the nuchal translucency in multiples of the normal median, the nuchal transluce ncy and crown-rump length in millimetres, or the reported prior and nuchal translucency-based risks.