At around 16 weeks i had an ultrasound, but my obs wasnt able to tell us the sex because Ben's "legs were crossed". So at 19 weeks we tried again, and found out our little baby was a boy. She was 98% sure. I think its much more difficult to confirm a girl, because there is always a risk that the boy bits could be hidden. Apparently a boys bits looks like a hot dog, and the girls, a hamburger... And im pretty sure the only way they can tell, is the penis (or lack-of).