I know that there are literally millions of other names out there.... Unfortunately there is only one my partner and I agree on!

Will wait until she finds out ( no point bringing it up now and finding out she's having a boy) and then talk to her and be up front, that's what I would want if I were her so I will do the same.
I reckon I'd mention it before. That way if she does have a boy and you have a girl she doesn't get angry you 'stole' her name as she may plan to use it in the future.