Veronica if your results from the NF and blood test come back lower than 1:300 you will be asked if you want a CVS or amnio done it also depends on when you get the NF results back.
The amnio involes them sticking a needle in to you tummy while doing a u/s to make sure they don't touch the baby and they get a small sample of amniotic (sp?) fluid and test it. The results take 2wks to come back and you will know if your baby has DS or not also you can get to find out the gender at the same time.
If you have any more questions feel free to email or PM.
I had the amnio done at 16wks.