Although most people think that the scans and tests are just there to identify Downs Syndrome, they actually show up a whole host of genetic abnormalities. The scans show up abnormalities regardless of whether you have the blood tests... Most genetic disabilities have a whole host of problems that can easily be identified on a scan and do not require the blood tests.
I had the blood test done as I was told they needed to be done to go hand in hand with the nucal fold scan... but I immediately turned down the amnio which I was told I should have due to my family history and risk factors.
I don't feel worried. The only thing I really wanted to make sure of is that my child didn't have Edwards Syndrome which is what my sister died of. I would have had a termination as to spare my daughter going through what I went through. But babies with severe genetic disabilities, don't move like ordinary babies, dont gain weight properly, have kidney/heart/lung problems.. all of which are visible on an ultrasound.
Down's isn't that serious, and in the cases where the genetic malformations are not that severe you can have babies born with mild forms of Downs without knowing... but having siblings with the condition and with quite serious cases of it, then I know I can easily handle that.
Besides there are many other things that can go wrong with your child which don't show up on the tests or the ultrasounds, like autisim or what my daughter has, so its no guarantee that you will have a perfect child, its just nice to get a heads up on these things...
having a kid is stressful enough... and if you wouldn't have a termination anyway regardless of the results of the tests... why bother putting yourself through it if you don't see it's necessary.