Talia - I have a close friend & my MIL who both birthed a baby with Trisomy 13. It is not compatible with life, but both made it to term.
My friends DD lived for 6 days & MIL's DS for about 3 days.
Neither of them knew til birth. My friend was young & didn't seek medical care very early, I think thats why she didn't know.
My MIL never went back to her doctor again, because from his physical appearance it should've been picked up on an u/s, but she was not told in any way that something may be wrong.
I just want to wish you well. I really don't know what I'd do in your situation. Can they do a normal u/s now & see if they can see any markers for problems now, before you decide what to do?