it also depends on doctor's experience. when i had first scanning in week 12, dr chan told me that IS a boy; i didn't believe. two weeks later i checked up in dr fung clicnic who said SHOULD BE a boy. now my boy is 28 months old.
Well, for my baby boy, I was sure it was a boy at week 14 or so - though the doctor did not confirm but we both saw the 'organ' in the ultra-sound. However, I guess if it was a girl, it's hard to be that certain at that time.