I didn't go for my first ultrasound until I was 18 wks pregnant. At that time, I told them I wanted to know the sex of the baby. The technician because of their policy and lawsuits did not tell me right there but told me my obstetrician would let me know the sex at my next check-up.
I found out from my obgyn that I was having a boy. Again, they will not
guarantee it becuase of a possible lawsuit if they are wrong.
I'm sure they could tell the sex at 12 wks, but it really depends on the position of the baby. Just ask. Also, when I went I brought a VHS cassette tape and had the ultrasound taped (this was in 2002), not sure if things are a little more modernized now.
When you go for your ultrasound just hope for a healthy baby, that's the most important thing.