Lucia, I think you can get an amnio at 15 - 18 weeks and it takes two to four weeks to get the results back. You can also have a CVS, which is performed earlier, usually 11 - 13 weeks or so, and with that, you get your results between 3 days and 2 weeks after. I had a CVS with both of my pregnancies (one daughter w/o CF, one daughter to be born w/CF) and both times I knew beforehand around the 14 week mark. If you do opt for the CVS, make sure it is with a doctor that has a lot of experience performing them. The miscarriage rate for CVS procedures is about 1% overall, but it is very dependant on the dr's skill and proficiency. A good doctor can perform a CVS with the same miscarriage rate as an amnio (.5%).
As far as preventing an unborn baby from having CVS, I wasn't aware it was possible. We were told that this still is not possible, but I haven't heard of PGD. What is that?