I'm going to guess that it depends some on the skill of the sonographer as well. LOL, we've done some informal scans at work with our portable US machine (we have 3 nurses who are expecting this spring!) and while we finally saw something on one girl and guessed right, it was just a guess with our untrained eyes
But I think the sonographers our two OB practices employ are pretty good- we do about 800 deliveries a year, and the majority- 80 to 90%, I'd say- of those know the sex from their 20 week scans. We only have a handful of ultrasound surprises a year- and when they happen, we all hear about it because they make some pretty good stories! The best one-
my labor preceptor, an OB nurse of some 20 years experience, had a delivery. This was right before she started orienting me to labor. They were expecting a boy, baby was born, no one checked. Baby was delivered to mom's chest and went to breast right away. The baby was called down to admitting as a boy. Parrents were calling baby by the name they had picked out for the boy they were expecting. Mom & baby were stable and happily breastfeeding and the nurse came out to the desk to give them some private time in between assessments. It was about an hour after the birth when the dad came running out to the desk saying "we have a problem!"
It seems someone finally got a glimpse of what was between the legs and it wasn't a boy at all!
Since then, I always have mom or dad check and annouce the sex immediately after the birth