Advice needed please

Hey ladies I had a question I am going to have my first ultrasound appointment with my first child and it's something I want to share with my husband alone and when I told my mom the appointment date she said she will come too (did not ask if she could ) and I couldn't say no (she's got a heart disease and is rarely excited over anything ) but I've been so frustrated about this  I really don't want her there I want to be alone with my husband. She's a 1st time grandmother and excited but it's something I need to do with my husband alone, what would u guys suggest would be the nicest best way to tell her no