Am going to go against the grain here and suggest that you don't say anything about this specific issue- the main reason being- what's the point? It won't change what happened, probably won't change anything in the future but will just cause even more negativity within your family. It might make you feel better for a little while, but from what I've read it will probably just get your MIL back up even more and give her reason (not that she needs any iykwim) to push your buttons even further.
About the 'our girls' crap - the best way to handle that one perhaps is next time she corrects you make a joke out of it saying, thats weird, I don't remember you walking around with an enormous belly for 9 months or something like that.
Most important consideration here is your girls (sorry, our girls!) and while I'm not trying to disregard your feelings in any way, shape or form, but by you keeping the peace on matters you can give your girls a lovely family childhood. What I mean here is with a little perspective on the things your MIL says/does are annoying, frustrating but don't really need to impact upon your girls and their relationship with their grandmother, and the family dynamics, I think you should let things go.
My memory of childhood family gatherings is just awesome, I have 10 cousins on each side and we would run around having an absolute ball. Wasn't til I was a teenager that I realised the level of tension in the air, and saw for myself that my fahter's mother was horrid to my mother, and my mother's brother was horrid to my father! They dealt with things in their own way and never did it impact on us. I only hope I have the strength of will to do the same, as sometimes after a couple of hours with my MIL I'm ready to stick a fork in my eye!
When your girls are older and your MIL says something outrageous, you can find a nice way with your girls to roll your eyes at each other with a look that says 'oh look, nanna's being a nutbag again,' without personalising it so much. I'm not suggesting let things slide that are deeply against your parenting, beliefs etc but the little things. And I'm also not suggesting that what you experienced when your cherubs were born is a little thing either, its massive, but its done and can't be reversed.
Sorry about the short story here but was thinking about your post and wanted to give you a good response, if that makes sense?
All the best,