A.P.
Remember that these are only screenings and they are only about 60% accurate if that. Your doctor should have explained that to you before you agreed to take the test. Amnios come with some risks also and you need to be VERY sure that you want this done. When I was pregnant, I opted to not get the triple screen (or quad) done because I had no intention of having an amnio and figured since the acuracy is so low I didn't want to stress. As it turned out, they found a choroid plexus cyst on my baby's brain via u/s and I was told that this could be nothing or it could be non-compatable with life. Nice options! It turned out to be absolutely nothing and I spent the next few months of my pregnancy worrying for nothing. I should have been enjoying this wonderful time! I think that all of this technology is great but no one tells you how much worrying it causes.
Bottom line...the stats are stacked FOR you in these cases. Do what you feel is best but ASK many questions to you OB as well as other OB's to get a feel for what's what. Some OB's feel that those 3-D ultrasounds can detect abnormalites safer than an amnio. (there are certain physical characteristics visible with certain chromosomal disorders.)
Another thing that you have to keep in mind is that the amnio, at best, can determine if there is a chromosomal abnormality...but it CANNOT detect severity! I have a friend whose son has Down's syndrome and he is HIGHLY functional and a GREAT kid!!