I had a very high ratio for down's syndrome too. Was told to do amnio test as well but me and hubby decided against it. No history of dear son in the family but I was 35 when I gave birth. I decided to go and do a detailed scan where they measure every part the baby and the doc told me after the scan that he was 100% sure that my baby was ok. Can't remember the doc's name but he is Indian and practices in Camden Medical.
Now that I have a beautiful girl, I really feel I wasted the $500+ I spent on that scan although it did out my heart at ease.
If you are young and there's no such history in the family, I think no need to worry.
There are a lot of cases where the results show high risk but baby is ok.
Just my two cents.