emkana- do you have access to counselling? It's such a difficult decision, especially if you have no experience of disability or termination. It's like choosing between 2 unknown awful situations. When I was pregant with ds3 we (dh and I) talked for the first time really about what we would do if markers for DS came back positive (previous pregnancies we'd thought we would go ahead). I insisted on the conversation as already having a child with a severe disability I thought it needed to be thought through.
Anyway when making difficut decisions I'm a great believer in taking the 2 choices and looking at them equally and then you kind of get a feeling for whichever decision is right for you. As a process of doing that I came to the decision that although I could terminate early (pre- feeling movement), I just couldn't once I had felt the movement and for me going through a late termination would be worse than actually dealing with a disability. Maybe I was seeking security in the known (ie the disability- which I knew I could deal with whereas I wasn't sure I could deal with the reality of a losing a well established pregnancy).
Anyway point of that ramble is not to say what I think you should do, but just that as an exercise (considering the 2 worst scenaraios and imagining living with and through both of them) it was useful for me as it made my decision very clear. I knewthat there was actually only one scenario that I could live with, so hey presto that was the decision, and once I'd made the decision I kind of relaxed (which I think is always a sign that its the right one).
I think it is important to try and come to a decision before you get the results, because you need to make the right decision for you (whatever it is), and once you get the results if they are positive it is often assumed that you will terminate (which is fine if you want to, and probably helpful, but can be very difficult if you don't want to or are undecided). Eidsvold found out that her dd was very likely to have DS during her pregnancy, so if you wanted to talk to someone who made that decision she's the one for you.
I would also have a think about what you will do in the maybes. I had a friend who went through similar, and when the results came back her ds did have a chromosome abnormality but saomething that was unlikely to cause any major problems, except potentially with fertility (last I heard he was a normally developing 4 year old). Obviously that case is fairly clear cut, but there are other shades of grey such as mosaics where the effect on the child can be very very hard to predict. I think its worth talking with your dh about your feelings there before the results because you may suddenly be landed with a situation you haven't considered and asked to make a decision in a very short time.
I hope that doesn;t sound a bit "do this and it'll be OK you'll know" as I know its not that simple, but I think when you are panicking it is very hard to clear your mind enough to know what's right for you.