@Kj1010 it's very unlikely not to be picked up at the 20 week scan so please don't worry about your results. But at the same time the NHS test does NOT test dna so it really is just age plus the nucal measurement and pap a. It produces many many false positives, as well as false negatives, but those are obviously incredibly rare and much rarer then false positives. But yes, there have been babies born to women who have had 1 in ten thousand chance of a baby with down sydrome on their nhs test. As well as babies born to women who have had 1/100. It is all about what you would personally do so if you would continue with the pregnancy then to be honest I would ignore the NHS results. Neck folds can be huge and the baby fine, or tiny and then at the 20 week scan something is picked up. The nipt is the only DNA test that gives you a yes or no answer with a 0.01 percent error margin. But still that is not diagnostic like the amio or CVS.
The NHS were supposed to be exchange the tri screening for the nipt because of its accuracy and it creating so much worry for women who's babies are absolutely fine. Equally , though in very tiny numbers admittedly, there is always going to be a few women who are given very low risks who get to twenty weeks to be told actually your baby does or may have Edwards or downsydrome and they need to do tests then.
Then there are the few where it is never picked up due to the spectrum of down sydrome as some cases are far more severe then others and less likely to be spotted on the anomonly at 20 weeks. But those chances are like one in hundreds of thousands!
If you are worried you can have the nipt, but it will cost you 400 pounds so it's a lot to no doubt confirm your baby does not carry any abnormalities or chromosomal issues