i would look at this in this way:
1. if it is the case, then it's a good idea to get the names and numbers of specific community leaders. now, because i had no u/s, i didn't have this situation, but "just in case" i had numbers on hand for families with children with special needs so that i would have community support soon after the birth if needed. btw, my son does not have DS or any other disabilities.
2. as for the testing, the real question is to know the value of the test.
if you do feel that it is likely ds and you also are concerned about specific problems to the infant that might occur during birth or shortly after, then i would continue with medical care and tests to make sure that the baby's needs are met. if it is possible, for example, to have a prenatal heart surgery that could change everything for the baby, then i would ocntinue with testing, etc.
but, more often than not, this is purely for information and peace of mind. there isn't a lot they can do until the baby is born, and you basically have the choice of knowing and bearing a child with ds (or whatever else might have been diagnosed) or choosing to go a eugenics route. personally, i find the second option to be repugnant. and i'm not sure that i would want a more risky test "just to be sure" either way, when there isn't much that they could do about it anyway.
those are just my thoughts on these things. take them or leave them.
but either way, good luck with everything!