B.Y.,
It is true that a grandmother should never forget the exciting news about her coming grandchild, but it's par for the course with AD. Your mother cannot help it. So sad, but true.
I was sure that my mother could NEVER forget me, her only child, yet that did happen. Just 6 weeks after she was living alone and driving her car, she couldn't say how many kids she had when she was asked. She said, "I don't know, they just keep showing up." I will say that even after I was 'too old' to be her Barbara and then she became in her mind 'too young' to even have a child, Mom was always glad to see me. She'd smile even when she could no longer speak a whole sentance and after she could not speak at all. When Mom forgot me, it was startling, but not the heartwrenching trauma that I had imagined it would be.
Take care of you and you deal with this awful disease. Do I remember that you have a 2 year old? If so, take lots of pictures of your child and your mother soon. When the new baby comes, take more pictures. In the future, the kids will be glad to see pictures of this grandmother holding them.
Wishing you well - Barbara