Hi OP - I'm wondering whether it would sound derisive to congratulate you on your pregnancy anyway? I'm sorry you face this situation.
I observed a Down Syndrome kid at the airport a couple of days ago, relaxed and happy, and found myself thinking about the fact that people with Down have become so much rarer since I was young, b/o all the screening and presumably resulting abortions. In a way I think it's a loss for society; it reduces tolerance for people with different abilities and needs than the norm and robs people of the opportunity to learn how to communicate effectively across the boundary established by intellectual disability.
My best friend growing up was friends with a girl with 'mosaic' Down Syndrome, so relatively high-functioning. Her condition was very visible in her face and her behaviour, but she was fun to be around and well integrated. That said, if she'd fallen among uncharitable adolescents, it could have been horrible. She died long ago of a heart defect that was part of her condition.
And I've also observed a couple of mothers not coping with a child with Down, which was horrible, and I know people in my own family whose attitude to ppl with Down Syndrome is vile. You'd have to deal with that sort, too.
As per pps, there's no right decision in principle, just what is right for you. Feels right, is doable. And of course your intuitions may not be very clear either. I hope you can take enough time over this - not sure what the deadlines are for abortion in this scenario. All the best xx