Hi - so sorry you are going through this, I had a very similar experience this pregnancy and it has been horrific. I can only share my experience as everyone is different but I did find it helped hearing from people who had similar experiences. The horrible thing I found in particular was that there is no quick answer; it becomes a process of crossing things off a list of things that could be wrong, that tends to drag on.
At my 12 week scan my NT was 4mm. I was told by foetal medicine that with a measurement of 3.5-4.4, the chance of a “healthy normal baby” was 70%, chance of chromosomal abnormality was 20%, and 10% chance of a structural defect like a heart problem. So there is a higher chance of all being well than not and often these things come to nothing - so it doesn’t mean there is definitely something wrong!
For me, with bloods and the NT, my chance of Down syndrome was 1 in 2 (lower chance for the other 2, 1 in 800 or so). Your odds of DS are far better than mine were.
We went abroad the next day for 2 weeks and I didn’t want to wait 2 weeks for another test and Would have missed the window for cvs so we paid for a private extended nipt that evening before we went (looked at Down, Edwards, Pataus, sex chromosome disorders and some microdeletions). I was completely expecting a high chance of downs from the Nipt, given the 1 in 2 chance at screening - but my NIPT came back low chance for all.
We saw foetal medicine at 16 weeks and they made clear that there are hundreds of thousands of things that could be wrong that the NIPt doesn’t look at and we were offered an amnio, which could rule out approx 20-30 syndromes. At this point the early anatomy scan they did looked completely normal, fluid had gone and no abnormalities identified - so I decided to sleep on it and wait for my foetal echocardiogram and anomaly scan at 20 weeks (again with foetal medicine) to make a call on amnio, given the low chance NIPt and good scans. I wasn’t keen on the risk of miscarriage associated with amnio as I know of people it has happened to, and I have previous experience of being in the unlucky 1% of stuff where pregnancy is concerned - although the risk is very low and I was probably overly cautious.
Foetal echocardiogram and anomaly scan came back completely fine, no issues identified - so we decided to leave the testing there. I’m now 30 weeks and have since had another scan at 28 weeks, again all was normal - growth all as expected, no structural issues identified. I won’t know until the baby is born unless I have a late amnio which I’m still debating but certainly the structure/growth of the baby looks normal which the midwife says is promising, albeit not a guarantee.
What I will say, and it’s totally personal, is that I wish I had ruled out more stuff by having an amnio. But I’ve spoken to other mums on here who have had similar experiences and some say they didn’t find amnio that reassuring tbh as there are many syndromes it can’t detect. I’ve found foetal medicine pretty good but definitely a bit doom and gloom and it feels a bit negative - but then they have to give facts and prepare people for worst case scenarios I guess. all I can say is that I really know how you feel and I felt exactly the same but that things have felt a bit more positive as time has gone on and now I feel a bit more philosophical about it and what will be will be. I still have very anxious days and I think that will continue until the baby is here. But I certainly don’t think it means there is definitely something wrong and there are plenty of stories on here with happy endings from a high NT measurement.
Not sure what your feelings are about parenting a child with Down syndrome but I joined the Positive about Down Syndrome Great expectations group on Facebook for expectant mums with high chance results and it gave me another perspective on what it might be like to have a child with DS - it doesn’t seem as scary as I initially thought. Obviously this is totally personal and it may be an absolute no for you.
Really thinking of you and sending lots of love, and hope all works out for you - there’s a perfectly good chance it will! Xx