Well, that's the tricky part. They don't tell you with 100% accuracy that your child has .down's .syndrome from the blood test. They tell you that your result was
abnormal and refer you for a detailed ultrasound and/or an amniocentesis. They don't diagnose your child as having .down's. They give you a percentage or ratio of your child's chances of having the disorder.
But you're right when you say that a large number of positive results end up with perfectly healthy babies.
Because of that, .i'm hesitant about aborting these fetuses. There's a fairly good chance that they are perfectly healthy. But as I am pro-choice, I would leave it up to the parents to make the decision as to whether they will go forward with the pregnancy or not.