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Nuchal Translucency

This is an ultrasound test in which the thickness of the fluid accumulated in the region of the base of the fetus's head is measured. In order to obtain maximal accuracy, a vaginal electrode (inserted into the birth canal) should be used to perform the test.

Why is nuchal translucency testing performed?

During fetal development, between the 9th and the 14th week, fluid accumulates under the skin in the scalp and nape region. In cases where there is excessive fluid (usually a fluid band of greater than 3 mm width) there is an increased risk of a fetus with both chromosomal defects (e.g. Down syndrome) and anatomical defects (such as severe heart defects). This test can detect women who are at increased risk for having infants with chromosomal or structural disorders.

When is the test usually performed?

The test should be performed between 11th and 13th weeks of gestation. If it is carried out at other times, it is difficult to compare the results with those of the women who have normal fetuses.

What is the significance of the result?

There is a correlation between the thickness of the fluid in the region of the base of the fetus's head and the risk of having an infant with chromosomal disorders.

There are tables that convert the results of this test together with the age of the mother into the degree of risk for having an infant with such a disorder.

As a rule, a nuchal translucency of 3 mm or above indicates that the risk of having an infant with a chromosomal disorder is increased more than tenfold for each age group.

The degree of the ability of the test to identify affected fetuses varies from one medical center to another (at a rate of 30% to 90%), but it is estimated that this simple test can usually identify 50% of fetuses with Down syndrome. If the result of the nuchal translucency test is integrated with the calculation of the first trimester biochemical screening, more than 80% of fetuses with Down syndrome can be identified.

What are the advantages of nuchal translucency testing?

The measurement of nuchal translucency is a simple, non-invasive test that does not endanger the pregnancy (as opposed to amniocentesis or chorionic villus sampling). If you have not yet decided that you wish to undergo amniocentesis or chorionic villus sampling, this test may help you with your decision (for example, if a very high nuchal translucency result is obtained, you may well decide to undergo invasive testing because of the increased risk for Down syndrome). Because the test is performed at a relatively early stage of pregnancy (up to week 14), if an affected fetus is detected, the pregnancy may be terminated by curettage without having to perform a late abortion involving induced labor.

The test is important mainly in multiple gestations. In these cases, it is the only test that is reliable for determining the risk for chromosomal syndromes. However, in all cases in which a high value for nuchal translucency is obtained and the prenatal test (chorionic villus sampling or amniocentesis) indicates a normal chromosomal structure, further testing should be performed, including directed ultrasound testing and fetal echocardiography (heart ultrasound), in order to rule out congenital defects and/or genetic syndromes. Hence genetic counseling should be offered in all such cases.

What are the disadvantages of nuchal translucency testing?

This test is usually not funded by the HMO, although it does not cost as much as the other tests. Also, its reliability is still at the research stage.

Prenatal Testing: Nuchal Test (Pregnancy Health Guru)


Nuchal translucency is a newly introduced ultrasound test that measures the thickness of the fluid accumulated in the region of the base of the fetus's head. The most reliable results are obtained between the 11th and the 13th week of pregnancy. The normal value in these weeks is up to 3 mm, and values in excess of this indicate a risk of congenital defects (especially heart defects) and of chromosomal syndromes in general and Down syndrome in particular. After the 14th week, the upper limit of nuchal translucency is 4-5 mm.

Nuchal translucency varies with the weeks of pregnancy, so in order to quantify the risk, the week of pregnancy and the size of the fetus in millimeters (CRL - crown-rump length) are taken into account. These indices together allow for the calculation of the weighted risk for Down syndrome. In the case of high risk (greater than 1:386), the mother is invited to come for genetic counseling, and the need for performing invasive prenatal diagnosis is discussed with her.

In cases in which a high nuchal translucency value is obtained and the prenatal test (chorionic villus sampling or amniocentesis) shows that the chromosomes are normal, a thorough ultrasound examination should be performed, including fetal echocardiography, in order to rule out congenital defects and/or genetic syndromes, and possibly other tests should be performed as well. In these cases, it is essential to provide genetic counseling.

Extremely high nuchal translucency is called cystic hygroma. This defect indicates the possibility of syndromes other than those described, requiring detailed counseling. Some of these syndromes cannot be tested for during pregnancy.
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