Just to let you know, the blood test can also show up risk of small for date baby. I was told I had 1 in 18 risk of Downs, I had CVS, and got negative result (no downs).
However, because of the dodgy blood result, they later sent me for a Doppler Scan. The results of that show that one of my arteries in the womb has a 'notch' which means blood flow isn't normal (and it's this which caused the earlier abnormal blood result, not Downs).
As I understand it, the risks are an increased chance of a small for date baby, high blood pressure and pre-eclampsia. They prescribed aspirin to thin my blood and are scanning me every 4 weeks to monitor growth.
If i hadn't had the test, I guess they wouldn't have picked up on this unless/until the baby actually showed physical signs of something being wrong. Instead they have been able to at least try to prevent it, and are monitoring me closely so should hopefully pick up first signs of trouble. I am also taking extra care over blood pressure.
I'm pleased I have the chance to try to do something about it now, and possibly prevent my baby being induced early because Drs think she/he's better out than in, IYSWIM.
Not saying what you should do, just thought you might like to know about this side of the tests.