This is spooky. This could actually have been written by me, except that I'm not actually pregnant yet. Are you me from the future?!
My mum remarried a few years ago. I was in my late 20s when they met so although I sometimes refer to him as my 'stepdad' for convenience, he's had absolutely no hand in raising me at all. I like him well enough and he seems to make mum happy but we're really not close.
Anyway, several times now my mum has brought up the subject of how she can't wait for grandchildren and how she and Jim (not his real name of course) have been discussing what they want be called. She tells me he has 'decided' that he wants to be called Gramps.
Now, I realise that it's ridiculous to be discussing this when there isn't even a grandchild on the way yet! But anyway, I once said that I always imagined he'd be Uncle Jim, and she got upset and said that would be very unfair and he'd be really hurt not to be referred to as a proper grandparent.
What I find odd is that my mum's father remarried when she was 19 and we have always, always referred to him and his wife as Grandad and Auntie Maureen. When I pointed that out, my mum insisted that it was different 
I think I eventually ended the conversation by saying 'well, names sometimes evolve over time don't they? And you can't always control what kids decide to call you!' and she seemed to agree with me on that.
I haven't lost any sleep over it but I'm not looking forward to reviving that conversation if and when I ever get pregnant!