I'd just ask her if she has any specific reasons for wanting to be called Mom Mom. It could be as simple as she doesn't want to be called "grandma" cuz it sounds old or something. You never will know her feelings on it if you don't ask. It could be just something she overheard and thought was cute. And, she may not realize that it's such a big deal to you. And, my only question is...is it that you don't want anyone to be Mom Mom to your son or that you don't want your mother-in-law to be Mom Mom? Meaning, a lot of times names like that get passed down...like if you called your grandmother a certain name, sometimes your child will call his/her grandmother (your mother)the same thing you called yours...what is your mother wanting him to call her? I hope it all works out for the best. I would definitely ask her, though, why she specifically wants that name.
Bookmarks