They do a 3 or 4 D special hour long ultrasound on you at a certain stage, take bw, and make a calculation. At that point, they either recommend or do not recommend an amnio.
I only know one person who has had an amnio in the last 5 years, because she was 40 when she got pregnant (naturally), and after the 3/4D thingy, they said she has a 1 in 40 change of downs. She did the amnio, everything was fine. She had a lovely little boy.
So, no, I figure if we ever are lucky enough to get to that stage, we will probably not have to do it. But heck, with my luck, you never know.