Welcome to healthcare magic.
Your estimated risk for having a Down Syndrome baby is above average but presence of polyhydramnios
, echogenic foci or a small chin is not an indicator of having a Down Syndrome baby for sure.
I suggest you to have two tests;
for karyotying of your baby ( this gives you a very accurate result about the genetic make up of your baby) & OGTT ( oral glucose tolerance test
to exclude diabetes
which is a common cause of polyhydramnios).
is far advanced & has reached viability state now. You have a choice of not to do any Karyotyping as at this stage it can only give you a more accurate answer about the genetic make up of your baby & you may be more prepared about what to expect ( at earlier stages of pregnancy parents may choose to terminate their pregnancy if tests are abnormal).
Hope this is a helpful answer for you.
Best of luck.