mumanddadof5, the scan and blood test are just a screening test, not diagnositic, so they just give you odds of having downs or other chromosonal abnormalities. They don't give you anything definite. This is why if the results are high risk (which depending on the doctor and your own situation, high risk could be anywhere from 1:6 to 1:300 or even more, it is an individual decision) a follow up diagnostic test, either amniocentisis or cvs is done. These tests are accurate but carry a risk of m/c. Many people choose to have the NT test as it can put your mind at rest if you get a result like 1:2000. But many women will get 1:20 or similar but have no problem, so this is a cause of stress as they need to decide on further testing and wait for it etc. So there are advantages and disadvantages to it.