First Trimester Nuchal Translucency Screening



Nuchal translucency screening is available to women between 10-6/7 and 13-6/7 weeks of pregnancy and is performed by a Fetal Medicine Foundation (FMF)-certified sonographer. Prenatal genetic counseling services are also provided in conjunction with nuchal translucency screening, if requested. This is a noninvasive screening technique where a measurement of the nuchal translucency is obtained to assess the risk of a fetal chromosomal abnormality.

What is the nuchal translucency screening test?

This screening test (also called the nuchal fold scan) uses ultrasound to measure the clear ("translucent") space in the tissue at the back of your developing baby's neck. This measurement can help your practitioner give you an assessment of your baby's risk for Down syndrome (DS) and other chromosomal abnormalities. Babies with abnormalities tend to accumulate more fluid at the back of their necks during the first trimester, causing this clear space to be larger. While it won't give you the definite diagnosis you'd get from more invasive test like amniocentesis, it can help you decide whether you want to undergo diagnostic testing. And unlike diagnostic tests, it's painless and involves no risk to you or your baby. Keep in mind this only a screening not a diagnostic test.

Ultrasound technicians (sonographers) and physicians need special training and high-quality equipment to perform it correctly, and they must be certified by the Fetal Medicine Foundation in London, the organization that currently sets the standards and provides the software that allows a doctor to evaluate your risk.

How is the screening done?

The doctor puts the nuchal fold measurement into a formula along with your age and the baby's gestational age and computes your baby's chances of having a chromosomal abnormality, based on statistical probability. Researchers have measured the nuchal folds of thousands of babies between 11 and 14 weeks of pregnancy. Because a baby's nuchal fold will normally get a bit thicker each day of gestation, researchers have been able to establish what an average or "normal" thickness would be for each day during those three weeks. Then they figured out 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 fold at a given gestational age the higher the chance of a problem and vice versa.

You may get the results right away or you may have to wait up to ten days if the doctor has to send the data to a processing center. You'll want to talk with your practitioner or a genetic counselor about how to interpret the results because unless you have a lot of experience with statistical analyses and evaluating medical tests, knowing what to make of them can be confusing. Even if you are a wizard with statistics, you'll need help understanding your options going forward.

What do the results mean?

You'll be given your results in the form of a ratio that expresses your baby's chances for having a chromosomal problem (based on your age, and his nuchal fold measurement). For example, if you're going to be 35 when you deliver, you probably already know that your baby's average risk for Down syndrome is 1 in 270. (This risk gets higher as you get older.) That means that one in every 270 babies born to women at age 35 will have DS. (One way to think about what this means is to picture yourself in a room with 270 other 35-year-old pregnant women - statistically, only one of you would be carrying a baby with Down syndrome.) If your baby's nuchal fold measurement is found to be average for his/her age, your baby's risk stays the same: 1 in 270. If it's thicker than the average, the risk goes up, and the baby's considered at a higher risk for an abnormality. If the nuchal fold is thinner than the average, his risk goes down.

Remember that this test doesn't directly test for chromosomal problems; it just gives you a better idea of your baby's statistical risk of having a problem. A normal result (sometimes called "screen negative") is not a guarantee that your baby is normal, but it suggests that a chromosomal problem is unlikely. Nor does an abnormal result (sometimes called "screen positive") mean that the baby has a chromosomal problem — just that he has an increased risk of one. (Even so, most "screen positive" babies still end up being normal!)

With the help of your practitioner or a genetic counselor, you'll then want to decide if the results indicate a high enough risk that you want to have more testing for a definitive diagnosis - that is, to see if your baby really does have a chromosomal defect. Individual parents-to-be have different feelings on what is an "acceptable" risk for them. Tests that can diagnose a chromosomal defect

What does it mean that this test is "80 percent accurate"?

You may have read that the results of this test are 80 percent accurate in detecting your risk of having a baby with Down syndrome. That means that if you're carrying a baby with Down syndrome, there's an 80 percent chance that the test will pick that up and give you a "screen positive" result that indicates further testing is recommended. It also means there's a 20 percent chance that the test will miss the Down syndrome and give you a "screen negative" result and diagnostic testing won't be recommended.

To be clear: This does NOT mean that a "screen positive" baby has an 80 percent chance of having DS! It just means that 80 percent of babies who have DS will have screening results that are suspicious enough to recommend diagnostic testing. And 20 percent of babies who have DS will be shown to be at normal risk — that is, the results will be falsely reassuring.

This screening test also has a 5 percent false positive rate. (A "false positive" result is what they call it when a test suggests there may be a problem when, in fact, there is no problem.) In this case, a 5 percent false positive rate means that 5 percent of all the babies with normal chromosomes who are tested will be "screen positive" — meaning that the test will show them to be at an increased risk even though they're normal. So based on this "false positive" result, their mothers may opt for invasive diagnostic testing that they otherwise might not have done. (and they will likely worry more; too, until a amnio shows that their babies are fine.)

What is the first trimester combined screening test?

It's a blood test done along with a nuchal measurement to give you an even more accurate risk assessment. A relatively new screening test called the first trimester combined screening combines the nuchal fold measurement with the blood test done at 11 - 14 weeks and the second blood test done at 16 - 17 weeks, increasing the accuracy of the risk assessment from 80 to 90 percent for Down syndrome. But remember, it's still an estimate of risk and not definitive like amniocentesis.

The blood tests measure two proteins in your blood: freeBeta-hcg (the free beta subunit of human chorionic gonadotropin) and PAPP-A (pregnancy-associated plasma protein-A). A few drops of blood are collected from your fingertips and results are usually available in 5 days. (Our center combines the results of some of these tests with the multiple marker screen at 15 to 18 weeks), which is the second trimester screen.

Finally, our center also includes one more factor into the risk assessment. We look for the baby's nasal bone. If it's not there at all between 11 and 14 weeks, the baby is at an increased risk for Down syndrome. Research results indicate that including this marker along with the mother's age, baby's age, nuchal fold measurement, and blood tests can bring the accuracy of the risk assessment up to 97 percent.

The nuchal fold test is noninvasive and carries no more risk than an ordinary ultrasound. And even if you forgo diagnostic testing, you can get more information about your baby's health and development by following up with a routine second trimester ultrasound at 18 to 20 weeks that looks for "soft markers" of chromosome disorders, such as short limbs, a bright dot in the heart, a bright bowel, cysts in a portion of the baby's brain, and certain problems in the kidneys.

What's the downside of these screening tests?

Like any screening tests, they're not diagnostic - that is, they can't tell you definitively if your baby has normal chromosomes. In some cases they'll cause needless intervention and worry, and in other cases they'll be falsely reassuring.

Also, the nuchal fold test doesn't detect neural tube defects such as spina bifida and other anomalies that may be indicated by the multiple marker test (done at 15 to 18 weeks), but the second trimester ultrasound should be able to detect these problems at least as well as the multiple marker.