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Nuchal translucency test
Definition
Nuchal translucency is the thickness at the back of an unborn baby's neck. Measuring this thickness helps assess the risk of Down syndrome.
Your health care provider will use ultrasound to measure the thickness at the back of the baby's neck. All babies have some fluid at the back of their neck, but in Down syndrome there is more fluid than normal. This makes the space look thicker.
A blood test of the mother is also done. Together, these two tests will tell if the baby could have Down syndrome.
Having a full bladder will give the best ultrasound picture. You may be asked to drink two to three glasses of liquid an hour before the test. Do not urinate before your ultrasound.
You may have some discomfort from pressure on your bladder during the ultrasound. The gel on the ultrasound wand may feel slightly cold and wet. You will not feel the ultrasound waves.
Why the Test is Performed
Your health care provider may recommend this test to screen your baby for Down syndrome. Many pregnant women decide to have this test.
This test can be done earlier in pregnancy than amniocentesis (another test that checks for birth defects). Nuchal translucency is usually done between 11 and 14 weeks of pregnancy.
A nuchal pregnancy test detects only Down syndrome. It does not detect neural tube defects or other birth defects.
Normal Results
A normal amount of fluid in the back of the neck during ultrasound means that it is very unlikely your baby has Down syndrome.
- A normal measurement at 11 weeks is up to 2 mm.
- A normal measurement at 13 weeks, 6 days is up to 2.8 mm.
What Abnormal Results Mean
More fluid than normal in the back of the neck means there is a greater risk of Down syndrome. However, it does not tell for certain that the baby has Down syndrome.
If the result is abnormal, other testing can be done. Most of the time, the other test done is amniocentesis.
Risks
There are no known risks from ultrasound. Ultrasound does not involve radiation, like x-rays do.
References
Simpson JL, Holzgreve W, Driscoll DA. Genetic counseling and genetic screening. In: Gabbe SG, Niebyl JR, Simpson JL, et al, eds. Obstetrics: Normal and Problem Pregnancies. 6th ed. Philadelphia, Pa: Saunders Elsevier; 2012:chap 10.
Reviewed By: Susan Storck, MD, FACOG, Chief, Eastside Department of Obstetrics and Gynecology, Group Health Cooperative of Puget Sound, Bellevue, Washington; Clinical Teaching Faculty, Deaprtment of Obstetrics and Gynecology, University of Washington School of Medicine. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M. Health Solutions, Ebix, Inc.
