K.S.
Two friends come to mind, the first had the same ultra-sound and when the baby was born, he was fine. At the ultra-sound, they are only looking at a physical characteristic that is typical in down's babies. In this case, the doctor thought it was nothing because the mom was built a certain way and thought that the baby was built the same way with a thicker back of the neck thing.
However, another friend has a down toddler. He is the sweetest thing. I always thought knowing before the birth is just wasted energy and didn't get any of the tests myself. However, after their experience I now know that it's important for the medical personnel to be prepared for a down's birth.
Good luck with the ultra-sound. My advice would be to not stress too much, that only the aminio can tell positively. Everything else is only an educated guess.