Hi D.,
Most 3D ultrasounds can detect gender bw 11-14 weeks. This is determined by a fetal medicine doctor at your first screening, but the technicians can usually see it too. I found out I was having a boy at 11.5 weeks! It is very common. good luck!
To add, I just remembered that it is called the "first trimester screening", and it must be done bw 11-14 wks for effectiveness, it's where they measure the nuchal translucency and check for a nose bone (indicators of normal vs. downs) and take a few droplets of blood to analyze your risk for various things. It is pretty standard amongst CA doctors, though all my pregnant friends throughout the US and Canada have had it done. It was routine for me. All my girlfriends decided to find out the gender during this process as well, The baby has to cooperate though! I wouldn't trust the mail-away blood-work. I would only trust it if my doctors were involved.