I have no experience of ds but we did have similar (at our 20 week scan) with trisomy 18.
The waiting is awful but hopefully they should be able to give you results for T13, T18 and T21 in a few days (although I'm guessing on blood work that they probably aren't worried about 13 or 18)
Have you heard of arc? They are a wonderful charity to support couples with antenatal results and choices. They have a fantastic helpline and you can talk to ladies who have been in your situation.
They've been a lifeline for me.
It is very, very hard and a real shock. I do know how you feel. T21 is compatible with life in lots of cases so you do have options.
Spend some time doing research, talk to people (like arc), cry and mourn if you need to and prepare to make a decision. I know that sounds awful and hopefully you won't have to but prepare yourself in case you do.
My dh and I had already decided with our first dd what we would do in the case of an incompatible with life diagnosis, we had also decided (roughly, you never know straight off in T21 cases what the issues will be) what we would do if we ever got a diagnosis of Down's syndrome. We never thought we'd actually have to make that decision but I'm glad we'd had the conversation before our results.
The flip side is that in a few days you get a call saying all is fine and you can celebrate. I really hope that's the case.