How do i talk him into it.

So my gender reveal ultrasound is on Wednesday. My mom has been so excited for this appointment and really wants to come with. She's been really looking forward to it and I want her to go. My obgyn allows as many people as I want in the ultrasound room, its a pretty big room and has enough chairs. So it was just going to be me, my fiance, and my mom. But my fiance just wants it to be me and him. I don't want to let my mom down, I'll be really upset and she will be upset if my SO doesn't allow her to come. My last appointment my brother, my cousin, my mom, my SO and of course me went. He was okay with that. But now he's all like no that's too many people it just needs to be me and you. I get he wants it to be special, but either way it's going to be special if my moms there or not. It's not like she's going to be in the way of anything, or loud. She just wants to come. I mean its my kid too so why don't I get to decide anything? Im really upset cause my mom will be upset and i know how special it will be for her. Its her first grandbaby. She loves getting to see my baby as much as i do. I really want her to come. Itd break her heart.

Am i in the wrong or.. how do i even convince him to change his mind. :/ this is something i really want and he refuses to let me have it.