I'm 21 and i didnt have the test with either of my two.
They will most likely pick something like that up at your 18-22wk scan anyways. Also ask yourself what you would do if ur bubs did have d/s.
I was having this baby regardless, my 20wk u/s gave me the all clear anyway.
Maybe ask about the blood test or something, sorry i dont know alot about it but thought i'd add in my 2 cents. lol

Good luck