The measurement of the fold is called the nuchal measurement and its size at 20 weeks is not relevant to DS. The measurement is only useful as an indicator of DS at the 12 week scan, it is a very specific time frame. After this time it is not at all an accurate indicator of DS. It *can* be a soft marker at 20 weeks and that combined with other soft markers may warrant further investigation, but it's not as strong an indicator as when done at 12weeks.

I'm not very good at explaining myself sometimes! Sorry.

A second opinion u/s is a good idea. I have had friends with similar stories and it was a false alarm each time. Second opinions are always warranted.