Vander, sorry you are having a worrying time.
We were told at our 20 week scan that our baby had two 'soft markers' for DS and for other more severe trisomies, and I did opt for amnio.
I think that the miscarriage rate for amnio has dropped since the introduction of better scanning equipment, as they can see better where the needle is going - but this is something to ask about.
Where are you? If you are close to s London you could ask to be referred to the Harris Birthright Trust Centre of Foetal medecine in King's, and I think there are other specialist NHS units around the country, with very experienced scanners and state of the art equipment.
They can indeed see more aspects that may act as 'soft markers' at the 20 week scan, but then soft markers are basically unproven slight tendencies in co-incidence that may or may not indicate a particular condition! The only diagnostic test is amnio.
Perhaps you could do some research into DS and children who have it, which may make clearer for you what you would do if you did know that your baby has DS. If you would not end your pregnancy anyway, you may not wish to have amnio, but having had a child with some leg abnormalities, I was very pleased that I knew in advance and had facts and support at the ready, rather than finding out when he was born.
I wish you good luck - I know full well that statistics mean very little once there is any chance at all that all may not be well with your baby. a millionth of a percent of a chance will fill the horizon with worry until you know what is happening.