I had a 1 in 22 chance for DS and chose not to have the amnio as we would not have terminated. Ultimately, our level II ultrasound at 20 weeks showed that there were no anomalies or soft markers. It is a very personal decision, but having had a MC last year, I couldn't bear the thought of terminating and I couldn't bear the thought of doing something to cause a MC. Which screen did you 1 in 5 chance come from? If it was a quad screen, please take some comfort that that test has a notoriously high false positive rate.
One more thing--with my first child, the quad screen returned a 1 in 10 chance for DS and I am happy to report that my firstborn does not have DS and is a healthy, growing 2.5 year old. It's hard to pass the quad screen if you're 35 or older...in case you happen to be that age. But a change in due date a couple days this way or that also makes a huge difference on this test too.