Omphalocele and potential T21

[NoCallerID]

I had posted this as a comment on an existing thread but no one replied and I realised I'm more desperate for any kind of experience in a similar position than I thought I was...

We had our NIPT come back positive for T21 this week, same day I also got the results from then nhs test with 1/40 HCG >4 and Papp-a 0.3, NT at 11+3 was 1.9mm. We were referred for a CVS which I had Thursday. At the scan it was confirmed that the baby also has omphalocele which was very visible at the 12 weeks scan at 11+3 but we were told to just come back in two weeks to check whether it has closed. Coincidentally I had come across an article a few days before the scan and I read that in around 40% cases this is linked to chromosomal abnormalities so although my world shattered when I got the calls on Monday I did have a feeling and was expecting the call.
I don't seem to find anyone on here (or maybe just didn't read properly) where they had a high risk NHS test, a positive NIPT and they still ended up with an all clear or mosaic result from the CVS?
We're not clinging onto hope and have made the decision to TFMR once we have the result from the CVS but I'm just wondering if there is a realistic possibility of getting a mosaic result back from it. The consultant kept saying this to us - before she had seen the omphalocele. After this her attitude changed a little and we felt she was a bit more understanding.... but she also said there could be an additional trisomy or it is not even T21 but a trisomy with similar markers and that's why it came up positive.
not sure what I'm expecting from this post. I guess I'm just lost right now. Thanks for reading.

@NoCallerID yes I got a mosaic result from CVS - following What turned out to be a scare where heart defect was suspected on ultrasound. It is possible, however as was stressed to me, less likely with ultrasound concerns.

If it helps to PM I am happy to share my experience in more detail with you. However my experience has been with Trisomy 18 so is a bit different. Mosaicism usually only shows on long-term cultures so I know the wait can be excruciating xx

Sorry, I'm just misreading this I think... the heart defect, was that a scare or actually there? I just don't have any hope with what we've seen on the scan, the consultant said there might be more abnormalities (heart & lungs) but the baby is too small to see yet...
I never thought I'd find myself in a situation like this. It's surreal and like I'm watching someone else go through it.

@NoCallerID I completely understand. It can feel like your world is caving in. The ultrasound I had indicated VSD and we were referred to Fetal Cardiology for tests a couple of weeks later (we were told the heart had to develop further for confirmation) which ruled the VSD was a false alarm. However at that point we also had the initial CVS suggest full Edwards and a 1% chance of survival. The prospects we were given were grim.

The ECG and mosaic results came through at more or less the same time. After that all ultrasounds were normal.

Wow that is crazy! I cannot imagine what you have been through!

Mosaicism is a lot more common than is often presented in statistics. It is only after researching extensively I learned that the latest research indicates mosaicism can be present in quite a large percentage of trisomies at CVS (trisomy specific I gather). However as trisomy is so rarely detected (i. e. Is not in the majority of pregnancies) it is technically accurate that mosaicism occurs in 1-2% of CVS samples.

The issue is that in terms of Positive Predictive Value, these figures are significant meaning that CVS is only accurately predictive of trisomy being present in the foetus in about 75% of instances. Hence why follow up amniocentesis should be recommended particularly when foetus appears healthy.

Knowing what I know now, I wouldn’t have a CVS or NIPT as they essentially only look at the placenta. In the case or NIPT it cannot even guarantee the cell-free DNA analysed is from the placenta at all - and does not test for mosaicism.

I honestly feel with both tests, the limitations of these issues are not adequately explained and discussed. Research over the last 30 years increasingly brings awareness to foetal-placental discordant findings.

Even amniocentesis has significant limitations, but it is more accurate than the majority of other tests.