My son had a nuchal thickness of 3.9 and with my blood tests I had a 1:20 chance of Downs. I know it's scary to think there may be something, anything, not perfect about your unborn baby. From here, you need to get the rest of the test results, and then perhaps choose an amnio if it matters to you to find out. I had the amnio and the results were fine. My son didn't have DS. All you can do is take one step at a time until you find out and decide from there. You might be worried for nothing yet.