Leave them out of it. It's amazing how many parents cannot see their children as grown-ups! I had the opposite problem - went thru infertility treatments and did NOT tell my father (who was battling cancer anyway and was difficult even when healthy). My brother decided my father "needed to know" about my treatments, told him, and then my dad had all kinds of opinions about stuff he knew nothing about.
So I think the thing to do is decide (and have your husband decide) that you guys actually know more about your family needs. A lot of older people look at disabled children as either an embarrassment or a reflection on their own genetic make-up, and they grew up when such children were shuttled off to institutions rather than mainstreamed and able to take advantage of so many therapies.
I would hope that, once you show up pregnant, or at least when you put a newborn in their arms, they will adjust! If not, it's their loss.
Meantime, just tell them it's not up for discussion. Leave the room or leave their home if you have to. I'm not sure there are any snappy comebacks that will help, although if they say you cannot handle any other challenges, you can say "Thanks for the vote of confidence." And then leave the room.
I think you can limit your visits with them too - if it's going to be unpleasant, tell them you are leaving. You can tell them it clearly upsets THEM to see you and discuss this, so you aren't going to bother them. Then stick to it. I'm not sure what your husband's parents' personalities are like - he may need a different strategy for them, or maybe he just needs to be pushed to stand up for himself.
Meantime, find other things to talk about. And DO NOT allow them to talk this way in front of your other kids - they may think grandma (or grandpa) doesn't like them much either. And I would tell grandma/pa this too!
Good luck!