We were told Sam was a boy at my 20 week scan, the lady said she was about 90% sure, as he wouldn't really show her the goods until right at the end of the scan, and even then he was a bit shy!
And then out came a little boy!
I'm pretty sure they are right most of the time, so you're pretty safe with all the pink clothes.
Although, I believe a certain gorgeous wee Kahtrell was thought to be a girl right up until the last minute.