As you can see from the other responses, the tests aren't always accurate. If you did the test and it showed that the baby was going to be DOWNS, then you'd worry the whole pregnancy, and the baby could turn out to be normal. Or, worse yet, you may decide to abort a perfectly, healthy baby. Or, the tests could show a normal baby, and then he/she is born with DOWNS.
Why take the risk, no matter how small? Why put yourself and your baby through unnecessary stress? I say...don't worry about it, let life be what it is, and however it turns out, you and the baby will handle things together. Just love, love, love that precious little baby growing inside of you. That is what's most important. Once you are finally able to hold your precious gift, DOWNS or NO DOWNS, life will continue as it is meant to. Just breathe...relax...and focus on what's really important.