Nuchal scan to detect birth defects?

I am beginning my second trimester and have just been scheduled for a "nuchal scan", a diagnostic test that my doctor says has replaced AFP as a more precise indicator of birth defects such as Down Syndrome and neural tube problems. Can you tell me anything about this procedure?


Peg Plumbo CNM

Peg Plumbo has been a certified nurse-midwife (CNM) since 1976. She has assisted at over 1,000 births and currently teaches in the... Read more

A nuchal translucency evaluation is a specialized ultrasound scan which targets the fetal neck for special evaluation. The reason this area is observed is because babies with Down syndrome have extra folds at the neck.

At this time, amniocentesis is recommended for genetic screening on all pregnant women age 35 or above. Because this is an invasive test with significant risks, providers and researchers have been researching other modalities to assess risk of genetic conditions and Down syndrome.

They have found that it is possible to assess individual odds of Down syndrome on the basis of multiple ultrasonographic parameters. One technique is to measure, via ultrasound, humerus length (the arm bone), nuchal (neck) thickness, degree of light reflection off the bowel (echogenic bowel) and shortened little finger. Likelihood ratios for Down syndrome are calculated on the basis of these parameters. In one study reported in the American Journal of Ob/Gyn, investigators were able to detect Down syndrome using ultrasound markers about 60 percent of the time.

Other studies have demonstrated that nuchal transparency may be a valuable marker for genetic defect and also of value in the diagnosis of other serious malformations, particularly cardiac defects.

At this point in time these tests are not requested unless a woman's age or history indicates a higher risk. I do not believe that these tests have positively been shown to be more accurate than the triple screen. Perhaps they will eventually be refined to the extent that they will be more predictive.

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