One of the mums at playgroup had a girl a few months ago and was told she was having a boy. She didn't buy anything new for the baby as she already had all the clothes she needed from her DS.
It was her 20 week scan she was told boy. She had to have a scan at around 36weeks and the couldn't get a clear look then either. I don't think she was ever that confident about what she was having.
If you really want to know I would book a 4D scan. That's what I did this pregnancy as once I found they thought it could be a girl, I wanted to know for sure!