What is the definition or description of: ultrasound for nuchal translucency?

Prenatal ultrasound: Usually done at or around 14 weeks, this test measures the thickness of a normal, fluid filled pouch at the base of the neck of the fetus. Large size correlates well with an increased risk of down syndrome. This test, which should be done by an experienced sonographer or doctor, can be paired with serum markers (sequential screen) to measure risk of other birth defects.

WebMD answer: "nuchal translucency test...Uses ultrasound to measure the thickness of the area at the back of the baby's neck. An increase in the thickness can be an early sign of certain birth defects, such as down syndrome. This test is often done along with blood tests in the late first trimester." see: http://www.Webmd.Com/baby/tc/birth-defects-testing-first-trimester-tests.

Nuchal Translucency : Nuchal Translucency is a screening test to determine risk of chromosome abnormalities in a fetus. When the NT is greater than 3.5 mm, there is a 1 in 5 chance of a chromosome abnormality such as Turner or Down syndrome, or Trisomy 18. Additional tests should be ordered by an OB or genetic counselor for confirmation. A mother may decide to continue or end of the pregnancy based on the information.