Just call her up and tell her.........email seems a bit unpersonal (she is, afterall, your mom), and in the waiting rooms is too late. Trust me, no matter how good your relationship with your parents is, you are going to have times when you need to put your foot down. Once you have kids, things change (obviously), and there are many times when you need to do what you (and your wife) think is right, not necessarily what other people think is right - this many times make others (such as grandparents) feeling upset, hurt, etc. For example, holidays, birthdays, random weekends, babysitting, etc. all present "unique" situations where things can easily get mixed/messed up. You might as well start making it known now, that it is your baby and you are the parents, and what you say goes. I am in no way condoning pissing off the grandparents, or anyone else for that matter, and I fully understand their excitement, joy, pride, etc., but along with many other things, your new baby will pull you in MANY different directions (and many different people). haha I hope I didn't confuse you, but my wife and I both had/have great relationships with our parents, but we still find ourselves getting in sticky situations like yours - it is unavoidable sometimes (you may say something, your wife may say something else - both innocent - and before you know it, you have someone coming over, someone's feathers are ruffled, etc.). Like I said, after 3.5 years and two kids, we still haven't got a good solution, but being up front and honest typically works the best - waiting to say something, or biting your tounge to many times, only leads to making things worse.
Good luck!