Names for grandparents

Courtney
So from the moment i told my mum and dad I was pregnant( was less then 5 was) I wanted them to be nanny and grandad, but my dad insisted on being grandpa, I didn't want him too but my mum said she thought was cute and persuaded me to let him be called it and decided she would be nana as I don't like grandma( plus my mum is only 43 and seems too young to be called grandma). I bought a bump to birthday book and have referred to my mum as nana for weeks. Now I'm 29 weeks and she's decided she doesn't like nana anymore coz the dog from Peter Pan, and everytime is brought up we both get annoyed. 
She says I let my dad choose what he wanted so I should let her. But she was the one who persuaded me into it and I HATE  Grandma( no offence ) 
The thing is I am very stubborn without all the hormones and just wondering what an outsiders opinion is. 
Am I in the wrong for not letting her change it? Even though I am so against the name. 
Sorry about the rant!