What Is The Nuchal Fold Test?

It's a risk statistic based on a non-invasive measurement made using ultra-sound (sound waves passing thru the wombs that can be interpreted to build up a 2D image).

The test is done between about 12 weeks and 13 weeks 5 days pregnant.  This is because all foetuses at this stage of pregnancy are very close in size (regardless of how big baby is when born).  So it's a good time to try to date a pregnancy, and to compare measurements of body parts for developmental reasons.

The nuchal fold is a fold of skin (and fat) in the neck.  In babies with certain chromosone defects the nuchal fold is much larger than usual.  The nuchal fold test has been especially calibrated to give the statistical risk of the foetus having the most common chromosone defect, which is Down's Sydrome, but it often also picks up other chromosone abnormalities.