I was 12 w 5 d when I had mine done. The fold measurement was 0.6 mm which put it right in the middle of the range (they showed me a graph) They used that result combined with the blood test result to give me a risk factor. Mine went from 1:360 for my age to 1:4000 something, which is a good sign that the baby is fine.

My OB told me that the next step (amniocentesis?) has a miscarriage rate of 1:200, so it would only usually be done if your downs risk came up as less than 1:200 if that makes sense.