metal our Down's test (the one where they measure the fluid in the baby's head and bloods) can back as 1:80. I was hysterical when they called me and told me, so I understand why you are so worried, but the odds are still very highly in your favour for everything being ok.
We were told on the Friday by phone. All the midwife said was 'high risk' and didn't give an actual number. We went to see the specialist midwife on the Monday and she gave us the figure of 1:80, which we worked out as something like a 98% chance that things would be ok. This made me feel a bit more positive.
I'd been on this board over the weekend and found out about the Harmony test and decided to go for it. It's 99% accurate for Down's results. It came back as a 1/10,000 chance i.e. 0.01%. We wouldn't have aborted for Down's, but I felt like I needed to be prepared if there was a strong chance of a Down's baby. The hospital arranged the test, but we had to pay £500.
All the best and I hope you get the same outcome we did 
ginger - so sorry to hear what you are going through. The Harmony Test tests for trisomies 13 and 18 as well as Down's, so maybe something to consider if you are worried about the amnio. Thinking of you and hope that everything works out as best it can 