The University of South Alabama Center for Women's Health is the first in Alabama certified to offer pregnant women a new ultrasound test that is approximately 80 percent accurate in detecting Down syndrome and other chromosomal defects. The noninvasive test uses an ultrasound to measure the nuchal translucency, or thickness of the skin at the nape of the fetal neck, at 11 to 14 weeks gestation.
The UltraScreen, a combination of nuchal translucency and a simple test using blood from the mother's arm, allows physicians at USA to detect more than 90 percent of patients at risk for Down syndrome and 97 percent those at risk for of Trisomy-18.
"Babies with abnormalities tend to have more fluid at the back of their necks during the first trimester, causing a 'thickening' of this clear space," explained Dr. Kathy B. Porter, professor and chair of the USA Department of Obstetrics and Gynecology. "While the nuchal translucency test does not provide an absolute or definite diagnosis, it does provide us an early screening with a high degree of detection without risks to the mother or child."
According to Porter, after reviewing the data provided by the nuchal translucency test, patients are better informed and in a better position to decide if they want to undergo more definite, but more invasive procedures. These procedures, which have been the standard of care prior to nuchal translucency, include chorionic villi sampling (CVS), an analysis of the genetic makeup of the cells taken from the chorionic villi - the tiny fingerlike projections on the placenta; and amniocentesis, a procedure that uses a needle to draw a sample of amniotic fluid.
In many cases, the nuchal translucency screening provides women an attractive alternative to amniocentesis and CVS. Amniocentesis, in addition to being invasive, causes miscarriages in about one in every 200 procedures. Approximately one in every 100 CVS procedures results in miscarriage.
The nuchal translucency test also provides convenience for patients. The entire visit can be completed in 30 minutes or less, with the ultrasound taking about 15 minutes and blood work another 15 minutes.
Research conducted in the early 1990s first demonstrated an association between thickening of the fetal nuchal skin and chromosomal defects, abnormalities of the heart and arteries and skeletal dysplasias, which is abnormal development of bone during development. Although still considered experimental in the United States , nuchal translucency is widely used and accepted as a standard of care in Europe .
The physicians and sonographers at USA are also certified in nasal bone testing, a variation of the ultrasound screening that examines the fetal profile for the absence of the nasal bone. This expertise increases the accuracy of screenings for certain types of chromosomal defects. The nasal bone is underdeveloped in 62 percent of fetuses with Down syndrome but only in 1.2 percent of normal fetuses. Nasal bone testing is performed at 15-22 weeks gestation and also reduces the need for unnecessary amniocentesis.
USA is certified by the Fetal Medicine Foundation based in London , England . Certification follows special training and demonstrated expertise in precise measurement of the fetus. The Fetal Medicine Foundation has set the standard for the precise way this measurement should be made and interpreted. Certification is granted only after additional training, demonstration of accurate measurement and submission of measurement data to ensure quality of care.
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 assist in providing an assessment of your baby's risk for Down syndrome (DS) and other chromosomal abnormalities.
Babies with abnormalities tend to have more fluid accumulated at the back of their necks during the first trimester, causing this clear space to be larger. While it won't give you a definite diagnosis, as you'd get from more invasive tests like CVS and 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.
This test has been performed in the U.S. since 1995, primarily at large medical centers, but it's still not widely available. Ultrasound technicians (sonographers) and physicians need to have 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 the doctor to evaluate your risk.
How is the screening done?
The scan must be done when you're between 11 and 14 weeks pregnant (the last day you can have the scan is the day you turn 13 weeks and 6 days pregnant). The sonographer will first confirm your baby's gestational age. (This is done by measuring your baby from crown to rump to see if he's about the size he should be for his age.) Then the sonographer positions the sensor (called the transducer) over the abdomen so that the nuchal fold area shows up on the monitor and measures the thickness of it on the screen with calipers.
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) so they could 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.
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, his 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.) If it's thicker than the average, his risk goes up, and he's considered at a higher risk for an abnormality. If his 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.
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 include chorionic villus sampling (CVS) and amniocentesis .
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 CVS or amnio shows that their babies are fine.)
What is this "Ultra-screen" test that I've been hearing about?
Some medical centers have added a blood test or two to the nuchal measurement to give you an even more accurate risk assessment. A relatively new screening test called the "Ultra-screen" combines the nuchal fold measurement with two blood tests (also done between 11 and 14 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 CVS or amniocentesis.) The Ultra-screen is not yet widely available but is becoming more common. 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). The blood samples are collected from your fingertips - they only need a few drops - and results are usually available in 24 hours. (Other centers may be combining the results of some of these tests with the multiple marker screen at 15 to 18 weeks.)
Finally, a few very advanced medical centers are also starting to include one more factor into the risk assessment. They're looking 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.
What are the advantages to nuchal fold and Ultra-screen tests?
The advantage to these screening tests is that they can give you a relatively accurate indication of your baby's risk for chromosomal problems at an early date without subjecting you to the small risk of miscarriage from a more invasive diagnostic test like CVS. If the risk is low, you can find out as soon as possible and may be relieved. If the risk is high you can decide whether to have CVS (done between 10 and 12 weeks), which can tell you for sure whether your baby has a problem while you're still in your first trimester. The nuchal fold test is non-invasive 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 test, 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.