D.W.
We opted not to have the quad screen with either of our children (though I was in my early 30's with both). There are high levels of false positives and false negatives with the tests, so we didn't want to get ourselves worked-up for results that were sketchy at best. And, by the time you have the test done, we were so attached to the baby, we would have figured out how to deal with the issues.
A former coworker had her 3rd child at almost 40 and had a positive Down Syndrome test. Her son was born perfectly normal.
Down Syndrome is caused by having extra chromosomal material on Chromosome 21. I don't know how accurate the tests are in determining this abnormality during pregnancy vs. other possible congenital abnormalities.
I do know, though, that most people who have a child with Down Syndrome consider them such a blessing. With 3 other children and no husband to help, I can only imagine the anxiety. I hope you find this to be a false positive and have a healthy, wonderful child.