Am I the only one annoyed by this?

You aren't being ridiculous about it at all. My mother did the same thing when I was pregnant with my first, as well did other members of my family. I did eventually snap and so far this time around nobody has done it again (except for my daughter, which is totally different). I think its just one of those things that some grandmothers do. I wish I knew why. My guess with my own mother is that the term grandbaby made her feel old, and this was her chance at feeling young again, and she had informed me several times that she was just too young to be a grandmother. I don't know if that's what it is with your mom, but I would try sitting down with her and really explain how it makes you feel. You don't want it to go on and then end up freaking out on her out of the blue.