MommyLizzy:This one I didn't find out till 20 weeks, but my first son I found out at about 15. Boy's are easy to find out though, if they say its a girl I wouldn't trust it.
Two of the women on my BMB were told 'boy' at an early scan and then found out they were having girls at the anatomy scan. So far, we haven't had any initial girl guesses turn out to be wrong.