Toward the end of your first trimester, your practitioner may recommend one of several prenatal screenings, including nuchal translucency.
These screening tests help determine the odds that a baby has a chromosomal disorder or congenital heart defect, although they can't diagnose these conditions.
Here's when and why nuchal translucency is recommended and what you can expect from this common screening.
What is a nuchal translucency test and what does it measure?
A nuchal translucency screening, or NT screening, is a specialized routine ultrasound performed at the end of the first trimester of pregnancy. It helps doctors determine if a baby is statistically more likely to have a chromosomal abnormality.
NT focuses on a small, clear space at the back of a growing baby's neck called the nuchal fold. Experts have found that this spot tends to accumulate fluid in babies with chromosomal abnormalities.
Babies with increased fluid at the base of their necks are at a statistically increased risk of having a chromosomal problem, such as Down syndrome (trisomy 21), Edwards syndrome (trisomy 18) or Patau syndrome (trisomy 13).[1]
All of these conditions involve having an extra copy of a specific chromosome (chromosome 21, 18, or 13, respectively). They can cause intellectual disabilities and mild to severe birth defects. The odds of a baby having a disorder increase with a mother's age, but they can happen to anyone at any age.
Keep in mind that nuchal translucency is not 100 percent accurate. It's a prenatal screening, which means it can't diagnose any condition.[2]
Because false positives with the NT screening alone are relatively common, it's often combined with a blood test to offer more insight into the relative odds of your baby being born with a genetic disorder.
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What is a normal NT measurement?
Normal NT measurements vary depending on how far along you are in your pregnancy.
In general, most doctors consider a normal NT measurement at 12 weeks to be under 3 millimeters.
Who should get a nuchal translucency screening?
A nuchal translucency screen is recommended for all pregnant women and is often one of several routine prenatal screenings during the first trimester.
It's ultimately up to you whether you have prenatal testing. The results can help you make prenatal care decisions. If follow-up testing diagnoses a chromosomal disorder, it can allow you to prepare for your child's special needs.
Although NT is widely available, some rural and urban areas may not have the resources to perform the procedure.
When a nuchal translucency screening is done
NT screening is usually done between weeks 11 1/2 and 13 1/2, but it must be performed between week 10 and week 13 of pregnancy. After that, the tissue gets thicker and is no longer translucent, so test results become inconclusive.
How an NT screening is done
An NT is a special type of ultrasound done using a very sensitive but safe machine. A sonographer will apply a transducer (wand) to the outside of your stomach to measure your baby from crown to rump and check that the fetal age is accurate. Then he or she will locate the nuchal fold and measure its thickness on the screen.
Those measurements, plus your age and baby's gestational age, will be entered into an equation that calculates the probability of a chromosomal abnormality. Although this number can't give you a definitive diagnosis, it can help you decide whether you want to undergo diagnostic testing.
Nuchal translucency and the first trimester combined screening
Nuchal translucency screening has a relatively high rate of error when performed by itself. To improve the accuracy of results, your practitioner will likely offer what's known as a combined screening.
In a combined screening, NT ultrasound results are paired with one or two blood tests that measure and compare your levels of hCG and PAPP-A (pregnancy associated plasma protein A). These two hormones are produced by the fetus and passed into the mom's bloodstream.
How accurate is nuchal translucency?
NT results by themselves have an accuracy rate of about 70 percent. That means that the test misses 30 percent of babies with Down syndrome or other chromosome disorders. Including the NT as part of a first trimester combined screening improves detection to between 83 and 92 percent.
Pairing the results of the nuchal translucency with other screening tests in the first trimester further improves the accuracy. The integrated screen, which combines the NT screening, measurements of the hormone PAPP-A, and the quad screen, improves Down syndrome detection rates to between 94 and 96 percent.
The results of an NT screen can also be combined with noninvasive prenatal testing (NIPT). NIPT is the most accurate of all prenatal screenings; by itself, it's 99 percent accurate at detecting Down syndrome. However, some insurance policies won't cover an NT if the patient has already had NIPT.
What if your NT results are abnormal?
If your nuchal translucency screening or any other prenatal screening results indicate that your baby may be at an increased risk of having a genetic abnormality, your practitioner will likely suggest a diagnostic test like chorionic villus sampling (CVS) or amniocentesis.
He or she may also suggest talking to a genetic counselor, who can better explain what the results mean.
Since abnormal NT measurements are also associated with fetal heart defects, your practitioner might recommend a fetal echocardiogram at around 20 weeks to look at your baby's heart. An abnormally thick nuchal measurement should be taken into account at the 20-week anatomy scan, with special attention paid to scanning the heart.
Increased NT measurements may also be linked to a slightly higher risk of preterm birth, so you may be monitored for that as well.
Keep in mind that all pregnancy screenings, including the nuchal translucency and the associated bloodwork, don't directly test for chromosomal problems. That means they can't diagnose any specific condition.
An abnormal result on the nuchal translucency or combined screening test doesn't mean that your baby necessarily has a chromosomal abnormality. They just mean that there's a statistical likelihood of having a problem. That's why your doctor will suggest follow-up diagnostic testing.
Try not to overthink your results. Many women who have an abnormal result on their screening tests go on to have perfectly normal, healthy children.
Also keep in mind that a normal result on a screening test is not a guarantee that your baby does not have a chromosomal defect, although it does mean that it's very unlikely.
How to prepare for a nuchal translucency screening
Experts say you'll get the best nuchal translucency results if you undergo the ultrasound with a full bladder, so try not to go to the bathroom right before you have your test.[3] The person performing the NT scan may also ask you to drink a glass of water before you begin.
Nuchal translucency risks
Both the ultrasound and the accompanying blood tests are painless (unless you count the needle prick for the blood test) and carry no risk to you or your baby.
False positives are relatively common with the NT screen alone, which may lead to follow-up procedures that present greater risk and unnecessary worry. But try to keep it in perspective: The odds of having a completely healthy baby are overwhelmingly in your favor.
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