I wonder how they are coping with the scan? I have to say, that for a 'normal' scan, it is unlikely that they will turn the screen away.
I can sort of understand that OP's DH is panicking and that is clouding what would be his normal, rational response, but I still feel that that is unfair on the OP, who is at the moment having to be the grown-u for her own DS's, be the grown-up with respect to her dsd, and even be the grown-up with regards to her DH. It's not what you expect to happen in a normal relationship.
I think some sort of family counselling would be a good idea, as maybe OP's dh will take more information in if he feels it is coming from someone impartial.
I can also imagine that the OP's dsd has not given a single thought to what happens 1/2/3 yrs down the line. I certainly didn't. And I certainly didn't imagine how difficult it would be to deal with a teenager when I'm only 29.
OP - You need to be making sure your dsd has some very good antenatal care if she decides to continue with the pregnancy, there are a lot of things that you have an increased risk of when you are under 18yo. If she is less than 12 weeks, she needs to be taking folic acid. She will need the triple test at about 16 weeks, as under 18yo's have a similar risk level of downs syndrome to a woman over 35. She needs regular checks on her blood pressure, as pregnant girls under 18yo are at higher risk of pre-eclampsia, and they are also at risk of premature delivery.