I had a different Trisomy 18 marker
Amber,
My story is a little different from yours but I thought I'd post anyway since I know a little of what you are going through.
My triple screen (I guess they didn't do quad screens 11 years ago) was fine but when we did the ultrasound, my son had choroid plexus cysts. These were fluid filled cysts that in and of themselves were not dangerous but there was some study that showed that babies that had the cysts at the 21 week ultrasound had a much higher chance of having Trisomy 18.
We did the high res ultrasound followup and he had no other markers for the disease.
Then we consulted with a neo-natal genetic specialist so that we could really understand the numbers behind the Trisomy 18 risk as well as the risk of harming a healthy baby with an amnio. We opted to skip the amnio because it seemed a much greater chance of hurting the baby with the amnio then that he had Trisomy 18. I'm not saying that getting amnios are bad in all cases, we just weighed both sides and chose not to. In your case, it might make better sense to go ahead and get the amnio and the peace of mind that comes with it. You should ask to have someone sit with you and explain all the risks on both sides.
My son was born perfectly healthy with no genetic defects. Something that my brother-in-law told me at the time (he's a pediatrician) is that with all these modern tests they do with pregnancies and percentages telling us the chance the baby has this defect or that, we are exposed to many more false positives and worries over nothing than ever before.
On the flip side, I have friends who had a negative triple screen and ultrasound and the baby was born with Downs. So, these tests can be unreliable in both directions (false positives and false negatives)
-- Mindy