Which scan was it? I've heard they're often wrong - we had hints it was a girl at our first scan but it's really too early then - next scan they said 'looks like a boy' (they tend to hedge their bets), so I kept asking at every subsequent scan - and he remained male!
No idea about stats but with ds I was told a boy at 17 wk scan and he was a boy (obv!) but with dd I was told at a 12 week nuchal fold scan (where, let's face it, they're looking at tiny measurements) 90% likely it was a boy too. It wasn't until the 20 week scan that the sonographer put me straight and said he was 80% sure it was a girl. Which she was!
I have 2 girls (or elves). With the first they couldn't say as she had her bottom facing the ultrasound, but they "didn't see any evidence of any boy parts". They said it was "probably a girl". With number 2 she was in the perfect position. The doctor showed me where there would be a penis if it was a boy, but in fact there was just an indentation. He left me to draw my own conclusions...
I guess it's easier to be sure if you see an actual penis, but in some instances the baby leaves little doubt that she's female.
i was told at my 20 week scan that they were 80% sure it was a boy. then i had to go and have a late scan as my belly was not measureing what it was ment to be and they told me they were 100\5 sure it was a boy!! and it was!!