The NT scan is an ultrasound done in the first trimester to determine your baby's risk of having Down syndrome and some other chromosomal abnormalities. It's usually done along with a blood test. If the screen indicates that your baby may have a problem, you can decide whether or not to have CVS or amnio for a diagnosis.
What is the nuchal translucency scan?
The nuchal translucency scan (also called the NT scan) uses ultrasound to assess your developing baby's risk of having Down syndrome (DS) and some other chromosomal abnormalities, as well as major congenital heart problems. It's offered to all pregnant women, along with a blood test, in first-trimester combined screening options.
The NT scan measures the clear (translucent) space in the tissue at the back of your baby's neck. (This is the "nuchal translucency.") Babies with abnormalities tend to accumulate more fluid at the back of their neck during the first trimester, causing this clear space to be larger than average.
Some practitioners also look for the presence of the fetal nasal bone during the NT scan. If no nasal bone is seen in the developing baby, some experts believe this increases the likelihood of chromosomal abnormalities. In some very specialized testing centers, other major abnormalities may be detected during the scan.
The NT scan is not invasive, and it's not conclusive. Diagnostic testing such as CVS or amniocentesis are necessary to confirm a positive screen result.
When is the NT scan done?
The NT scan must be done when you're between 11 and 14 weeks pregnant, because this is when the base of your baby's neck is still transparent. (The last day you can have it is the day you turn 13 weeks and 6 days pregnant.)
Are there other tests to screen for Down syndrome?
Cell-free DNA testing (NIPT), which can screen for DS as early as 10 weeks, is also available to all pregnant women. This blood test is 99 percent accurate in detecting DS, but women who get a positive result would still need CVS or amniocentesis for a sure diagnosis.
If you've had NIPT, you would probably not also do an NT scan, because it would be somewhat redundant. In some testing centers, however, other genetic abnormalities (such as anencephaly or an absent kidney) might be picked up on an NT scan that were not screened for with NIPT. You could also choose to have an NT scan without the blood work, if you already had NIPT screening.
What else can the NT test tell me?
In addition to testing for Down syndrome (trisomy 21), the NT scan will also screen for:
- Trisomy 18 (Edward's syndrome)
- Trisomy 13 (Patau syndrome)
- Some other chromosomal abnormalities
- Some structural problems, such as heart, abdominal wall, and skeletal defects
If your baby's nuchal translucency measurement indicates that he's at increased risk for structural defects, you'll be given a special ultrasound (also called an advanced level or level II ultrasound) and a special test called a fetal echocardiogram, both in the second trimester. If your baby has a structural defect, he'll need to be monitored with regular ultrasounds and delivered at a medical center equipped to handle his condition.
What are the advantages of first-trimester prenatal screening?
First-trimester screening lets you learn about your baby's risk for chromosomal problems relatively early in the pregnancy without subjecting yourself to the slight risk of miscarriage from an invasive test like CVS.
If the risk is low, the results will offer you some reassurance. If the risk is high, you may want to consider CVS, which will reveal whether your baby has a problem while you're still in your first trimester.
How is the nuchal translucency scan done?
The sonographer first confirms your baby's gestational age by measuring him from crown to rump to see whether he's about the size he should be for his age. Then she positions the sensor, called the transducer, over your abdomen so that your baby's nuchal translucency shows up on the monitor and measures the thickness of it on the screen with calipers.
How are NT scan results calculated?
Your baby's chances of having a chromosomal abnormality are determined by the nuchal translucency measurement, your age, your baby's gestational age and the blood test results.
Your age is factored in because although anyone can have a baby with a chromosomal abnormality, the risk increases as you age. For example, your likelihood of carrying a baby with Down syndrome ranges from approximately 1 in 1,200 at age 25 to 1 in 100 at age 40.
Because a baby's nuchal translucency normally gets a bit thicker with each day of gestation, researchers have been able to establish how large the translucent area should be each day during the three weeks the screening can be performed.
They've also calculated the statistical relationship between this measurement, the baby's age, the mother's age, and the likelihood that the baby will be born with certain abnormalities. In general, the thicker the nuchal translucency at a given gestational age, the higher the chance of a chromosomal problem.
How soon will I have the NT test results?
You may get the results right away, or you may have to wait up to 10 days if the doctor has to send the data to a processing center.
What do the NT test results mean?
You'll want to talk with your practitioner or a genetic counselor about how to interpret the results, which can be confusing. Many centers will report each individual lab result as well as the final calculated result using a special formula.
You'll get your results in the form of a ratio that expresses your baby's chances of having a chromosomal defect.
For example, a risk of 1 in 100 means that for every 100 women with this result, one baby will have Down syndrome and 99 will not. A risk of 1 in 1,200 means that for every 1,200 women with this result, one baby will have it and 1,199 will not. The higher the second number, the lower the risk.
Normal or abnormal results
You may also be told that your results are "normal" or "abnormal" for a particular condition, depending on whether the ratio is below or above a specified cutoff. For example, some tests use a cutoff of 1 in 250. So a result of 1 in 1,200 would be considered normal because the risk that there's a problem is lower than 1 in 250. A ratio of 1 in 100 would be considered abnormal because that risk is higher than 1 in 250.
Remember that a normal screening result (screen negative) isn't a guarantee that your baby has normal chromosomes, but it does suggest that a problem is unlikely. Likewise, an abnormal screening result (screen positive) doesn't mean that your baby has a chromosomal problem – just that he's more likely to have one. Most screen-positive babies turn out not to have a problem: Only about a third of babies with an increased nuchal translucency measurement turn out to have chromosomal defects.
How accurate is the NT scan?
The NT scan alone will detect about 70 to 80 percent of babies with DS (depending on which study you look at). The detection rate for the NT scan plus a first-trimester blood test ranges from 79 to 90 percent. (Most tests will include both the NT scan and the blood test, unless you've already had NIPT testing and opt to only have the NT scan at this time.)
This does not mean that a screen-positive baby has a 79 to 90 percent chance of having DS. It just means that 79 to 90 percent of babies who have DS will have screening results that are suspicious enough to recommend diagnostic testing. And 5 to 21 percent of babies who have DS will be determined to be at normal risk — that is, the results will be misleading.
False negative result
Screening tests aren't perfect. They don't detect all cases of DS, so they may identify your baby as being at low risk when she really does have DS.
In other words, if you're carrying a baby with DS, there's a 79 to 90 percent chance that the combined screening will detect the abnormality and give you what's called a screen-positive result indicating that further testing is recommended. But it also means there's a 5 to 21 percent chance that the tests will miss the DS and give you a screen-negative result.
This is called a false negative result, and it might lead you to decide against diagnostic testing that would have revealed a problem.
False positive result
These tests also have a 5 percent false-positive rate. A false-positive result is one that suggests that your baby is at increased risk for DS or another condition when, in fact, there is no problem.
What happens if a nuchal translucency scan indicates a problem?
With the help of your practitioner or a genetic counselor, you'll want to decide whether the results indicate a high enough risk that you want to have CVS or amniocentesis to get a definitive diagnosis.
In making your decision, you'll need to weigh your need to know about your baby's condition against the small chance that diagnostic testing could cause a miscarriage. Individual parents must decide for themselves what risks are acceptable.
If you decide not to have diagnostic testing, you can get more information about your baby's health and development by following up with NIPT, the quad screen (a blood test in the second trimester) as well as a detailed ultrasound at 18 to 20 weeks.
This ultrasound can detect "soft markers" of chromosome disorders, such as short limbs, a bright dot in the heart, a bright bowel, and certain problems in the kidneys. It can also look for anatomical defects, such as spina bifida.