I had the Nuchal Translucency test done at 12w6d I was told it has to be done between 11w6d and 13w6d otherwise they cant get the correct reading. The sonographer and the doctor said everything was completely normal. The doc said that they count the folds in the back of the neck and check the density of bone. The test is optional unless you are told it needs to be done obviously. But i think its worth it, you dont have that constant worry hanging over your head the rest of your pg.
Have a great day
Mel