Hi OP did you specifically ask for antenatal screening? Because if so they will give you a likelyhood or "risk" of having trisomies (Downs, Edwards, Patau Syndrome) and they work out that risk by checking the scan (Nuchal Translucency, the build up of fluid behind the baby's neck) your age (or egg donor age) do you smoke, do you have diabetes and also the combined bloods.
The bloods are testing your Free Beta HCG and your PAPP-A levels.
All this information combined will give you a risk factor.
The background risk (for example, the risk factor for everyone before the screening) is 1:1200 for Downs and I think even less for Edwards and Patau.
With my combined screening I came back as a 1 in 87 chance of Down Syndrome.
If you get a result like this they will ask you if you want further tests. The test that the NHS offer is called an Amnio which is invasive and has a 1% risk of miscarriage. Because of this me and OH opted for private screening which cost £300 and it's a blood test. That test was 99.7% accurate.
We had to wait a whole week after the scan to find out the results and thankfully we came back all clear and the baby was fine. And the perk of the test (because they are looking at the baby's DNA) they could tell us the sex at 13 weeks 
we are having a little boy!
I would advise anyone to think seriously about whether they want the screening because I literally worried myself sick for a whole week and sank into depression waiting for those results.
If you come back as low risk however and the scan looks fine then I'd say your good to go and spread the good news!
My high all came purely on bloods by the way, my HCG was four times what It should have been! I'm 26 so age wasn't a factor. They still didn't explain my dodgy bloods. It's a forever mystery. Good luck x