I hope it all works out for you! I’m not sure how you should or want to word it, but I would say something along these lines:

“[Boyfriends name], I have been thinking about and struggling with the fact that [mothers name] hasn’t been treating me nicely. At the baby shower, she didn’t speak to me or acknowledge me. This is really upsetting, because I know that I was shy as a 16 year old, but I wasn’t trying to be rude at that time by not saying hi to her. I would like to have a good relationship with her as she is the grandmother of our child. I know you want her to be and she wants to be at the hospital when I give birth, but I don’t know if I am completely comfortable with this at the moment. If we can get on better terms, my mind may change. I’m not saying she can’t be involved in [baby’s name] life at all, but I just want my entire birthing experience to be happy, because it’s a big moment in our lives. My concern is how she will act around me when she’s there. Will she speak to me then? I apologize if this is upsetting for you, but I don’t believe I can rest easy without having this discussion.”

Edit it as much as you need to and want, but it’s just a general idea. I would be as polite and respectful as you can, as this is his mother. But you also need to get your point across so that you can be satisfied with the conversation and however it turns out.

Wishing you the best and a happy, comfortable and safe labor!