Girl at 15 Weeks, Maybe a Boy at 18 Weeks

We went for an elective ultrasound at 15 weeks,

it was a 3D ultrasound and she confirmed that we were having a girl. Pictured is the picture from that ultrasound. Yesterday we went for our 18 week ultrasound with our regular doc, 2d and he thinks the baby is a boy. So basically he said he was unsure. How can this turn into a boy? I'm totally confused. I have no pictures from the 18 week ultrasound of the genitals. We are going for another elective ultrasound tomorrow to confirm.