We had someone who insisted DS2 was going to be a girl even though we clearly saw his boy parts on the ultrasound and had no doubts he would defiantly be a boy. Luckily I didn't have to see this person that often and the rest of the family were happy to accept that he was going to be a boy.
Hopefully once you have the scan your mum will stop saying silly stuff like that. Hope you get some nice photos of your little man :)