@lubbhates, what kind of down syndrome test you are taking? Oscar test or harmony test?
Down Syndrome test is quite different from detail scan at 20 weeks.
Down syndrome is to test the probability of trisomy 21. If you do the oscar test, they will scan your baby, measure the thickness of the fluid at the back of your baby's neck, nasal bone, etc combined with the blood test then come out with the probability of having down syndrome baby. If you take harmony test, they will only take your blood test, no scan at all. However, harmony test is more accurate compare to oscar test, but the price for harmony test quite expensive though.
While detail scan on 20 weeks would checking the anatomy of your baby, they called it fetal anomaly scan.