If your doc said there is a possible risk for DS, then to know for sure the next step is to have an amniocentesis done. Its optional and risky but if you want to know, that would be the next step in testing. Good luck
The doctors offices have alot of false markers for down syndrome. They end up doing lots of ultrasounds and usually find nothing wrong. I had a marker for about 10weeks but my blood was fine, and after doing research online, I realized they are some what common and cleared. Its possible the tech didn't get a good angle for the baby also. Stay positive :) if the blood comes back normal, I really wouldn't worry at all. Take care