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
I think it is different in different places. Where I went the NFT had to be below 1 in 200 before they asked you about amnio or CVS because that is the point at which the liklihood of a miscarriage resulting from the CVS or Amnio equals the chance you have something wrong. In other words if you were assessed as having a 1 in 300 risk of down syndrome from the NFT - then the risk of miscarriage from the CVS or amnio is higher so they don't recommend you have it.