Blood test.

Im going to get a blood test to check for Down's syndrome, trisomy 18, spinobifida, & other genetic disorders. They say from this blood test they can tell you the gender, this blood test gets done between weeks 11-13 im 13w3d so I have til Friday to get this done. Anyone get the wrong gender from this? I still have my ultrasound for almost 19 weeks but just wanted to know how accurate it would be? & do I need to ask to know the gender before they take the blood or is it just something they know from it?