Regardless of all the testing available, even if you've had an amnio or CVS which shows no abnormalities you can still get a surprise on the birth day and have a DS bub.

How is that possible? The amnio and CVS are chromosomal testing and they'd notice if the extra chromosomes were in the result.
If you get a positive result (meaning a baby has DS or one of the many other trisomies) that they test for then this is 100% that the baby has it. If on the other hand you get a "no abnormality detected" it means just that - no abnormality detected - it does not mean 100% that your baby will not have a trisomy - it means that it is extremely unlikely that it will have them that's why they use "no abnormality detected" - to cover themselves just in case something in the test did not work. Extremely rare to have that happen, but it can happen.