What happens in a 13 week NT scan?

I know this may sound stupid but I am being sent for a 13 week NT scan to check for Down Syndrome, Trisomy 18 and Neural Tube Defects. In my unborn Child.. I was told by My cousin who just had her baby that the way they do this is with a needle through the belly.. I am very scared I dont know what I got myself in to. This is my second child with the first one I didnt do any of this, Please Ideas will help.... thanks