The nuchal translucency measurement (+/- the blood test results), along with the woman's age and the estimated age of the baby (measured by the ultrasound) are combined to calculate a 'risk figure', or the chances of the baby having a genetic disorder. The result may be given as just 'low risk' or 'high risk' or you may be given the neck measurement and a specific figure expressed as an 'odd'. A 'low risk' is considered more than 1:300 (for example, 1:400), a 'high risk' is considered less than 1:300 (for example, 1:30). These estimates are explained fully in class B, understanding 'the numbers'.
A nuchal translucency is only a screening test, it aims to identify babies that may be at increased 'risk' (or have a higher chance) of having a genetic disorder, but it cannot definitely tell if the baby does have a genetic disorder. Nuchal translucency alone is about 75% accurate, but if it is combined with the blood test, this increases the accuracy up to 85%. This means that around 15 to 25% of babies with a genetic disorder such as Down Syndrome will be missed, meaning the woman will be estimated as 'low risk' and yet still carry a baby with a genetic disorder. About 5% (or 1:20) women will be estimated as 'high risk', yet most of these babies will be very healthy and normal (this is called a 'false positive result').
NOTE:Despite the level of inaccuracy in all screening tests, a nuchal translucency + the blood test is widely accepted as generally being the most accurate in predicting affected babies, especially when compared to the triple blood test, which was more popular before the use of NT.