I need HAN's help on this one.

So my wife got us into a sticky situation. Here is the situation. We are going to find out what the sex of our baby is going to be this week. :yay: But my Mom asked my wife if she could come with us when we find out. Because my wife is such a people pleaser she said yes, to my Mom, even though she only wants it to be her and I at the doctors. Now I am left to inform my Mom that she can't come.

What I am wondering is how should I do this? Would it be rude to write her an email? Or is this something that I should tell her in person? I would prefer to email her because she can be very manipulating and I don't want to have to deal with that. But I feel like I should tell her that she can't come in person. I don't know what to do. Anyway's help me out with this one. Thanks.