I was told at 12 weeks ds1 was probably a boy and it was true - he was.
However I was also told at 12 weeks dc3 was probably a girl - at 20 weeks it was quite clear it was a boy.
They gave the usual caveats "can't be completely sure" etc however the sonographer said quite definitely he thought it was a girl. At twenty weeks we told the (different) sonographer this and she said, whilst scanning, "but would you mind?" "oh no not at all" we said, so... "good, because it's a boy!". Luckily it was true, we didn't mind, if anything slightly favoured a boy.
DC2 (who was a girl) they said at 12 week scan that they couldn't tell yet.
So, I wouldn't take it too firmly. 12 weeks is very early to be able to tell. Do you have much of a preference either way?
And most importantly I guess, if this is your main train of thought after the scan, the baby is fine? Congrats on that