I went and had my pg confirmed by my GP today (well actually Itold him I was pg and he said "Congrats, where are you gonna go?", but that's another story.) Anyway, we spoke of the scan/bloodtest thing which is new since DS was born in June 2005.

Apparently, it's the combination of factors from the blood and the results of the scan which give a more accurate ration for downs. Let me get the paperwork so I can quote...

What does the screening test for Down Syndrome involve?

1. A blood test
2. An ultrasound

By combining the results of the 2 parts of the test, it is possible to identify if a pregnant woman may be at increased risk of having a baby with Down Syndrome. By itself, the test does not tell you whether the baby has DS or now. It identifies women who should be offered furthr testing to determine if their baby is affected.
Part 1, the Blood test.
The blood test, also known as teh maternal serum screening test measures the amount of two different proteins called PAPP-A and beta HCG which appear naturally in the mother's blood during pregnancy. A change in the level of these proteins may indicate that tehre is an increased risk that the baby has DS. Previously a similar test was available but only after 14wks pregnancy.
Part 2, The ultrasound.
The ultrasounds can be done by a specially trained untrasonographer between 11wks 3 days and 13wks 6days of pregnancy...
The ultrasound allows measurements of the amount of fluid in the skin at the back of the baby's neck. This measureemnt is called teh nuchal transluciency.
All babies at this stage of pregnancy ahve some fluid in this area, but on average a baby with DS or another chromosome abnormality has a larger amount of fluid. The NT measurement can therefore be used to identifiy if a baby has an increased risk of DS.
...
How accurate is the test?
By combining the knowledge of the woman's age, the results of the blood test and nuchal translucency measurement, the test can identify about 9 out of 10 pregnancies in which they baby has DS. This is more accurate than each test done by itself.
I think that long winded explination can be summed up by the last line, but I didn't know that until I typed it all. Hopefully someone will find it all interesting.

HTH