Bride09> think they are the same...
This is a collection of fluid under the skin at the back of a baby's neck. It can be measured using ultrasound when your baby is between 11 and 14 weeks old. All babies have some fluid, but many babies with Down's syndrome have an increased amount (Snijders et al 1998: 351, 343-6).
A nuchal translucency (NT) scan is a screening test which assesses whether your baby is likely to have Down's syndrome. A screening test can only estimate the risk of your baby having Down's, whereas a diagnostic test, such as CVS or amniocentesis 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.