Oooof oh boy. Ok, first, I really don't think checking in with a call once in awhile is that big of a deal, as long as it's to see how they are doing and you are making it about them and their new married life....both of them, not just your son.
It's also good to invite to a dinner too once in awhile, or ask if you can do anything for them.
The Ooooof oh boy is my reaction, as others here, when you went up and asked her about being pregnant!! Even without the miscarriages, which btw make this even worse, it's very invading to their privacy. And besides, many couples just can't wait to surprise the to be grandparents with the news.....
You kinda killed the surprise! And imagine the pressure she feels, now everyone knows and if she isn't or miscarries again......I mean, it's just too personal of a thing at this stage.
I would apologise.....not to your son, but her. Make it clear you give yourself no excuses....you did something without thinking and you are very sorry, will never do something like that again.
When my son married, I made it a point to be my DIL's friend, which wasn't hard because I really liked her.....even after they split up at only 9 months married.
She even called me to explain after my son left her. Now, I did listen to my son's issues and just expressed how sorry I was it didn't work out for him (I was upset too, I liked her, but didn't focus on that)
She called up and explained. I told her what my son said and I listened to her side. I felt they both had issues, so I told her I was sorry she got hurt.
I actually think I would have remained friends with her, but she lived a little far and I figured with no kids, why do that, and I didn't want my son in the middle of it.
I hope you can aplogise effectively and try to become a friend to her. It is really like having another daughter. At least that's how I felt.