Accuracy matters. The ultrasound is a screening test, and as such may over or underestimate the presence of what is being looked for. Accuracy is often dependent on the training of the tech and or who reads the study. The blood test is a definitive test, it is either positive or not. I would never rely on the ultrasound for any decision other than pushing me to order the blood test.
Great question. Amniocentesis takes a fluid sample and looks at foetal chromosomes. Down's demonstrates trisomy 21; edwards syndrome, trisomy 18. Others too. An u/s tells nothing about chromosomes, and there is not a contradiction of one test by the other. The chromsome changes are not artifacts. If you do not want to know, do not get the test. Every couple needs to make their own choice; it's no one else's.
Too sensitive a. Question. It is not clear what you mean by blood test. If it is a screening test, then you need to get additional tests, like amniocentesis done for a definitive answer. Please discuss the matter with your doctor and I wish you and your baby the best.
High chance. Positive screening should be confirmed by karyotyping of fetal cells obtained through amniocentesis. Consult your obstetrician for further details.
Which test????????? The answer depends on what test was done.