The quad screen has a lot of false positives - that's why it's a screen. I haven't experienced this, but here's how we guided our decision making about all of the prenatal tests that were offered. Most important question to ask yourself is what will you DO with the knowledge. Will it affect any decisions you make? Would you decide to terminate the baby if it DID have T18? Or would you carry it to term and give it comfort care and let it die (or be stillborn)? Hard decisions, I know, but one that you may be facing. If it will give you peace of mind for the next 4 weeks, or if you would make a decision to terminate the baby based on the results, then do the amnio. Either way, we'll keep our fingers crossed for you and say a prayer - it can't hurt :)
For us, we did not choose to get screened for this very reason - we could not think of a situation that would make us decide to terminate the baby. On the other hand, we did get the US at 20 weeks because we decided that if there *was* something wrong we'd at least like a heads up so that we could be prepared.