20 week ultrasound. Could the gender be mistaken?

Alexis

This will be our 4th child. We have 3 girls. We are really hoping for a boy this time. The baby was in breach position so she couldn't get full measurements of the spin and the stomach measured smaller than it should because of the position so we'll need another ultrasound next month. She said the gender of this baby is a girl but its hard for us to see what she does. Could she be wrong? Could it be harder to tell in breach position?