When I had my 12 week NT scan I was told the cut off between high and low risk was approx. 1/300. They didn't have a moderate risk category. I don't know the process for detecting risk of DS in the second tri scan. My score was about 1/400 at 12 weeks which I had mixed feelings about, even though I was technically in the low risk I still felt at a moderate risk. IMO 1/300 is still a long shot of having DS. It could happen to any of us as the scans can't detect with absolute certainty.

I agree with Jungle Mum that an amnio could give more information in case she's thinking of terminating but it could also give some psychological reassurance or help to prepare. She probably should ask the risks of having an amnio as the risks might outweigh the benefits.