Good luck, its normal to be a bit nervous before your scan, they will do a measurment of the liquid behind the babies neck and combine that with your blood test to get your chances ratio.

You may have to wait a day or so for the results(being a friday, not until monday or even tuesday), and if your dr is slack like the one i went to- you may even need to chase them up for your reassurance, i had my scan last monday and didnt hear anything for a week, so i figired no news was good news, which was right, when i called for the 3rd time and picked up my films and read the report, my chances ratio was fantastic 1:3342, the lady who did the measurment was pretty confident that the measurment was good so i wasnt too worried, but if you dont want to sit around waiting once its done, tell your dr that you WANT a phone call regardless of the results, as the policy some drs have is "i will call if something is wrong" and that can leave you wondering for days!!!

Good luck!!