20 week anatomy screening/ mother in law

So in August I have my anatomy screening: finally get to find out the gender of our baby, I'm excited but also a little frustrated. My partner's mom lives kind of far away and she wants to go to the appointment with my partner and I to find out the gender. I've told me partner he needs to tell her no. I don't feel comfortable having her in there because if there was something possibly wrong with the baby I wouldn't want her in there. Also, I want my partner and I to find out the gender first and then tell our families. This is her first grandchild so I know she is excited, but I also feel like this is something just my partner and I should do together. Am I being too dramatic? My mom will not be going to the appointment. Any suggestions on how to tell his mom I don't want her to come to it?