An anatomy scan is very important, and you need to have one between 18-22 weeks, IT IS NOT A GENDER SCAN! but people normally find out the sex.
If you had an ultra sound before that it was for something else (dating pregnancy is very common, and I think its the NT test they do one for also) that doesn't count as your "one" U/S.
I have had 4 U/S so far and I'm only 21 weeks, It might even be 5 actually. If your doctor thinks you need one then you will get one, paid for.
Personally, I don't care about the sex I just want a healthy baby. And I was way Happier to find out my baby had a healthy heart, and 2 kidneys, then find out the sex. Thats what the anatomy U/S is for.