I went to Kingsbury and used Dr Shannon Morris for all three pregnancies. At the 12/13 week scan she knew each time. She was able to tell at 12 weeks 3/4 days on all three of them.
1st boy, 2nd girl, 3rd girl. She did not hesitate each in case, she literally scanned over the area, and when I said "do you know" she always said "yes."
She knew with certainty each time. I asked in each case, is there any room for error, should I leave a percentage of "let's wait for the 20 week scan" and she looked at me like I insulted her mother.
She knew in each case. When ever I got back to my OBGYN he never appeared surprised, though I never asked him at the time to "sex" the baby.
Obviously I confirmed it at the 20 week scan - "so, is it still a boy/girl?"
I am not sure if in all cases an OGBYN who scans in his rooms can see, but if your scan at a fetal assessment centre is being done by an experienced specialist they should be able to tell.
However, Dr Morris is a bit of a legend, so she knows stuff that maybe other doctors/scanner operators do not always know. She is very experienced.