As far as I know the down syndrome test is not invasive. So using u/s they observe if they can see excess skin in the backside of the neck of your baby and also check the nose bone. Then according to those results and your age and history they will calculate a probability of you having a down syndrome baby. Depending on that they will tell you if it's safe or if you need more testing, which can actually become invasive.
For other chromosomal abnormalities (as far as I have read) you do need to get some sample from the placenta which increases the probability of mc of 1-2% but only during a few days.
We are not doing the test as it wouldn't make a difference for us (we would keep our baby anyway).