Extracted from Babycentre:
"What is nuchal translucency (NT)?
This is a <u>collection of fluid under the skin at the back of a baby's neck</u> at 10-14 weeks that can be measured using ultrasound. All babies have some fluid, but in many babies with Down's syndrome, the nuchal translucency (NT) is increased.
An NT scan is a method of assessing whether your baby is likely to have Down's syndrome. It's a screening test. A screening test can only estimate the risk of your baby having Down's, as opposed to a diagnostic test, such as CVS or amniocentesis, which will give you a definite diagnosis (but also carries a small risk of miscarriage).
The NT scan can't tell for certain whether your baby is affected. However, it can help you decide whether or not to have a diagnostic test."