Nuchal Translucency Screening and Nasal Bone Screening

What does it find?

Nuchal translucency/nasal bone screening is a combination of an ultrasound measurement and two blood tests. It screens for Down Syndrome, trisomy 18, and congenital heart defects. Knowing the ultrasound measurements, the mother’s age, and the results of the blood testing, a higher or lower risk of these problems can be given, leading to reassurance, or further, more definitive testing.

What is Down Syndrome?

Down Syndrome is a leading cause of mental retardation and birth defects, and is caused by an extra copy of chromosome number 21. Although the likelihood of having a child with Down Syndrome increases with age, any pregnancy can be affected.

What is trisomy 18?

Trisomy 18 is another example of a condition caused by an extra copy of a chromosome. In this case, number 18. It causes severe birth defects and mental retardation, and more than 95% of affected fetuses die soon after birth.

How is the test done?

After answering any questions you may have, an ultrasound is performed in which an area of fluid accumulation on the back of the baby’s neck is measured, called the “nuchal translucency” (NT). It has been found that an increased NT is associated with a higher than normal risk of Down Syndrome, trisomy 18, and several congenital heart defects. A sample of the mother’s blood is then obtained for measurement of levels of PAPP-A and free B-HCG, abnormal levels of which can also indicate a higher risk than normal for these problems.

How accurate is this test?

When performed properly, the NT measurement by itself can find about 70% of fetuses with Down Syndrome and trisomy 18, and about 30% of fetuses with major heart defects. Adding the blood testing increases the rate of detection to 80-90% for Down Syndrome and trisomy 18.

When is it done?

NT/nasal bone measurement is measured between 11 and 14 weeks.

What does a positive result mean?

About 5% of NT screening will return positive. This doesn’t mean that the fetus is abnormal. However, it does mean that there is at least a 1 in 300 chance that the fetus is affected by Down Syndrome or trisomy 18, approximately the same risk as a 34-year-old woman. It is important to remember that the majority of “screen positive” pregnancies are normal.

What does a negative result mean?

A negative result does not guarantee a normal pregnancy, even for the problems being screened. However, it indicates that the chance is low for these problems. No other diagnostic tests would be recommended unless other risk factors are present.

If I have a positive test, what's next?

There are several tests available to help determine whether a positive test result is real. These include: (1) Chorionic villus sampling, (2) Amniocentesis, (3) Targeted ultrasounds, and (4) Fetal echocardiography. The advantages and disadvantages of these tests will be discussed with you if you have a positive NT screen.

Do I do more tests after the Nuchal Translucency Screening?

Although very useful for screening for several problems, as outlined above, this testing doesn’t detect open neural tube defects such as spina bifida. Another blood test, called AFP may be drawn, or an ultrasound at 17-18 weeks may be performed to evaluate for this.