The 'unborn' have no rights, in my view.
Yes, I would abort in this scenario: it is one thing for a child to develop a significant disability, but I see no reason to deliberately bring this onto myself, and more importantly, them, with all the challenges it will bring.
I have a cancer causing gene (SDHB), and it has been explained to me that, should I wish to have children, I can genetically select for those eggs without the mutation. I certainly would.
I also have a deafness gene (unidentified), and this is more of a grey area; but again, if I knew what one it was, I would choose to abolish it (I do o.k with a cochlear implant, but have been massively affected in many ways).
This does not mean I think it is the right choice for everyone. I have no doubt that people with DS can, and indeed to, live rich and fulfilling lives; but it's not a challenge I'm up for, and shouldn't be judged for that.