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 risks. Neither will be correct, and we describe how to calculate a valid combined risk. This uses the reported serum‐based risk and a likelihood ratio derived from the nuchal translucency report. Tables, figures and examples are provided to aid the calculation of the likelihood ratio from either the nuchal translucency in multiples of the normal median, the nuchal translucency and crown‐rump length in millimetres, or the reported prior and nuchal translucency‐based risks.