Hi OP, hope you're well. It's such a difficult time you're going through, I know I've been there. When I was pregnant with DS, we were told there was a 1in5 chance of him having Down Syndrome. We went for an amnio, and I found out on my birthday that he did have Down Syndrome. It was a massive shock and we felt extremely sorry for ourselves. I now realise, with a 5yo DS with Down Syndrome how I couldn't have been more wrong. DS was born with several heart defects, 2 holes and several leaky valves. However at 4 months old he had heart surgery and looking at him now, or even a few months after the surgery- you would never known he'd been so poorly. Yes, there is a a high chance of a heart defect, but many children with DS either have heart surgery early and it's all fixed or they end up never needing surgery as the hole gets smaller and almost insignificant as they get older/bigger. I also know a few children with no genetic differences who have holes in their hearts awaiting surgery (where the parents didn't know until they were born).
My 5yo DS with DS has been treated as if he can achieve and he has. He has had speech and language therapy and physio, he's now a very active little boy. He talks in full sentences, has a massive vocabulary, reads at a year 2 level (as he loves books), maths at year 1 level. But more importantly than all that, he is the absolute joy of our lives. Everyone he meets, he makes laugh and has this amazing ability to be positive in all situations. He has the most cheeky sense of humour, he makes me belly laugh daily.
A lot of the stories on here are of people with DS who are 40+. We know so much more now and early intervention is the standard. If you see a 40+ year old with DS, they wouldn't have legally been able to be educated in a mainstream school. If they were 50+, they may not have had an education at all. Medical sciences have come such a long way too, most physical issues can be corrected quite easily. The people who are 40+ with DS would never have been offered heart surgery as babies as they weren't seen as 'worthy'! Things are very different now, our kids with DS are showing the world how much they can really achieve if we just believe they can.
Down syndrome is not a spectrum as people say, but there is a higher likelihood that they may have other co occurring conditions that may make learning a little more delayed. E.g. They may have hearing loss or some visual impairment (both my DS and I have a visual impairment but we have adaptations that allow us to do everything 'normally'). Or they may have ASD that make their sensory needs more heightened. These things can occur in kids without DS too though!
After having a miscarriage and genetic testing, I found out to my massive surprise that I actually have a form of Down Syndrome called Mosaic Down Syndrome, where some cells have an extra chromasome and others don't. I had a speech delay when I was little and I have several physical issues (eye sight is poor, hypermobile etc) but I was able to go to mainstream and was treated like any other child. I disingaged at primary school and was told I would never achieve anything, but actually went on to do well in secondary (where the classrooms were quieter and calmer!) and then went onto gain a BA, an MA and a Post Grad from Oxford University.
I don't say this to make it sound like achieving high is the main aim for a child, because it's not. But I say it to make the point that a lot of what we think of when we think of Down syndrome is extremely outdated and the kids who are born today with it have so many more opportunities and support options than the adults with DS that we may know.
I highly recommend the charity Positive About Down Syndrome https://positiveaboutdownsyndrome.co.uk/
They also have an 'Expectant parents' group on Facebook (closed group) that is really supportive no matter which way you decide to go. I think they also have some free impartial therapists that you can talk to.
I also recommend the YouTube channel 'Attitude' which shows lots of families who have children with DS, just living their normal everyday lives. So it will give you a better picture of what to expect.
I hope that whatever the outcome, you can have some peace and enjoyment in your pregnancy, it's such a special time. Wishing you all the best.