"Without exception, every woman I know with grandchildren has been uncontainably exited as the due date gets closer, and then when the baby's born."
You've not met my mother then Shesparkles. 
Not everyone has a decent set of parents. My mother totally RUINED my breaking the news conversation with her TOTAL lack on interest in me, my pg (her first GC) and actually said (after he was born) that she never wanted to be a GM, but that now she was getting used to it and enjoying it. Somehow that bell can never be un-rung in my head. I was pretty much ignored throughout my entire PG, and when she came to the hospital to see me, took my DS's dad down the pub, so I was actually left with LESS visitors than I had before she rocked up. When she visited me at home, she just sat in the kitchen being waited on hand and foot, not actually pitching in to help.
No, I got naff all support, my DSis got the flying visits, the staying over for a few days, the drop everything service.
OP, if you are hurt, TELL her. It may not make any difference to the choices, but if that is the case, you need to learn now that she ISN'T the Mum you can rely on, and to make sure that you don't get yourself into a position when she is all you have.
When you have your own child, you will learn that your own childhood may not have been as perfect as if could have been. I'm hoping that your mum will step up, but i don't sadly think she will.
I'd fly by flapping my own wings to be there for my DS, there is no way I'd ever leave him to be abused, or favour anyone else above him, I'd give him all the support, encouragement and advice I could, if that is what he wanted.
there are threads here to help anyone with mothers like these, the Stately Homes thread is a life line, a great place to sound out things and get perspective.