Hi BB Rabit,
I googled, and below is an extract:
At 12 weeks of gestational age, an "average" nuchal thickness of 2.18mm has been observed, however, up to 13% of chromosomally normal fetuses present with a nuchal lucency of greater than 2.5mm, and thus for even greater accuracy of predicting risks, the outcome of the nuchal scan may be combined with the results of simultaneous maternal blood tests. In pregnancies affected by Down syndrome there is a tendency for the levels of human chorionic gonadotropin (hCG) to be increased and pregnancy-associated plasma protein A (PAPP-A) to be decreased.
Based on the NT, blood tests and mother's age, the computer will compute the risk ratio. The normal cut off as high risk is basing on a ratio of 1:250 chance, but the result technically is accurate up to ard 65% and if say any lower then 1:250 will normally recommended by gynea to do a amniotest at ard wk 20~22.