Like all expectant moms, Deborah Martin is excited and worried.
Because she's 38-year's old, Deborah's baby is at higher risk for some conditions like Down Syndrome. In the past, to test for it, she'd almost certainly face an amniocentesis, which she says she'd like to avoid for obvious reasons - it involves a big needle inserted into the belly.
The ultrasound she recently underwent could help her avoid the needle. It used the same technology to look at her baby in a different way.
Dr. John Evans of Stormont-Vail says advances in ultrasound allow technicians to finer features. What they look for now is a fluid pocket in the back of the neck that seems to be associated with congenital anomalies.
It's called nuchal translucency. It measures the clear space in the back of the fetus' neck in millimeters. The thicker it is, the greater the risk of abnormalities. It's done between 11 and 14 weeks - that's earlier than amniosentesis and less risky.
Dr. Evans says it doesn't involve invasion of the uterus. He says you're not approaching the fetus with a needle, which minimizes the risk of trauma to the baby and infection. Plus, he says it's a less expensive procedure.
If the scan, combined with a blood test, does find a potential problem, parents could follow up with amnio. Dr. Evans says amnio is still the "gold standard," but nuchal translucency can help help mother's avoid unnecessary amnio procedures.
Nuchal translucency's been done in England for about five years, but Topeka's only sonographer trained in it has been doing it here just a few weeks.
Deborah's glad to be among the first to take advantage of it. She says it's given her reassurance and everything with her little one looks just fine.
The scan might be recommended for expectant moms over age 35 or who've had a child with a congenital abnormality.
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What is the nuchal translucency screening test?
This prenatal 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. That measurement can help your healthcare practitioner assess your baby's risk for Down syndrome (DS) and other chromosomal abnormalities as well as major congenital heart problems.
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. While the nuchal translucency screening test won't give you the definite diagnosis 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 United States since 1995, mostly at large medical centers. But it's becoming more widely available as more ultrasound technicians (sonographers) and doctors get trained and certified to do the test.
Sonographers and doctors 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 sets the international standards and provides the software that allows a doctor to evaluate your risk. If you're interested, ask your healthcare practitioner or genetic counselor whether it's offered in your area.
How is the screening done?
The nuchal translucency screening must be done when you're between 11 and 14 weeks pregnant. (The last day you can have it done is the day you turn 13 weeks and 6 days pregnant.)
The sonographer first confirms your baby's gestational age by measuring your baby from crown to rump to see if 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 fold area shows up on the monitor, and measures the thickness of it on the screen with calipers.
The doctor will put that nuchal fold measurement into a formula along with your age and your baby's gestational age, so that your baby's chances of having a chromosomal abnormality, based on statistical probability, can be computed. Researchers have measured the nuchal folds of thousands of babies between 11 and 14 weeks of pregnancy. And because a baby's nuchal fold will normally get a bit thicker with each day of gestation, they've been able to establish what an average or "normal" thickness would be for each day during those three weeks.
They've also 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 chromosomal problem.
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, they can be confusing. And even if you're a wizard with statistics, you'll need help understanding your options.
What do the results mean?
You'll get your results in the form of a ratio that expresses your baby's chances for having a chromosomal defect (based on your age, his age, and his nuchal fold measurement). For example, if you're going to be 35 when you deliver, 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 35-year-old women 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 you're 35 and your baby's nuchal fold measurement is average for his age, your baby's risk for DS stays the same: 1 in 270. If it's thicker than the average, he has a higher risk for an abnormality. If his nuchal fold is thinner than the average, his risk is lower.
Remember that this test doesn't directly test for chromosomal problems — it just gives you a better idea of your baby's statistical likelihood of having a problem. A normal result (sometimes called "screen negative") isn't a guarantee that your baby is normal, but it suggests that a chromosomal problem is unlikely. And an abnormal result (sometimes called "screen positive") doesn't mean that your baby has a chromosomal problem — just that he's more likely to have one. In fact, most "screen positive" babies turn out to be normal.
With the help of your practitioner or a genetic counselor, you'll then want to decide whether the results indicate a high enough risk that you want to have more testing to get a definitive diagnosis — that is, to see whether your baby really does have a chromosomal defect. Individual parents-to-be have different feelings about what's an "acceptable" risk. Tests that can diagnose a chromosomal defect include chorionic villus sampling (CVS) and amniocentesis.
(If your baby's nuchal fold measurement is above the 99th percentile for his gestational age, he's also considered to have an increased risk for major congenital heart disease, so you'll be given another screening test called a fetal echocardiogram. If your baby has heart disease, he'll need to be monitored with regular ultrasounds and delivered at a medical center equipped to handle his condition.)
Can you give an example of someone's results?
Here's an example of how it might go: A 40-year-old pregnant woman starts out with a 1 in 77 chance of having a baby with Down syndrome. She comes in for a nuchal fold scan at 11 weeks and 2 days, and the sonographer finds that her baby's nuchal fold is very thin for his gestational age. When the doctor feeds the results (the nuchal measurement, the baby's age, and the mother's age) into the risk assessment program, he finds that her baby's risk is now down to 1 in 761. Her baby's risk for trisomy 18, another abnormality, is even lower — 1 in 10,000. The woman is relieved and decides that since her risk seems low enough, she won't have the more invasive diagnostic testing (CVS or amnio).
What does it mean that this test is "80 percent accurate" in detecting Down syndrome?
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 DS, there's an 80 percent chance that the test will pick that up and give you a "screen positive" result indicating that further testing is recommended. It also means there's a 20 percent chance that the test will miss the DS and give you a "screen negative" result, in which case diagnostic testing won't be recommended.
Note: 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.
The test also has a 5 percent false positive rate. A "false positive" result is one that 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 the babies with normal chromosomes who are tested will be "screen positive" — the test will show them to have an increased risk even though they're normal. Based on this "false positive" result, their mothers may go ahead with invasive diagnostic testing that they otherwise might not have. (And they'll probably worry more, too, until a CVS or amnio shows that their baby is fine.)
What is a first trimester combined screening test?
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 first trimester combined screening 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 first trimester combined screening isn't available everywhere, but it's becoming more common. The blood tests measure two proteins in your blood: freeBeta-hCG and PAPP-a. A few drops of blood are collected from your fingertips and results are usually available in 24 hours. (Some centers combine the results of some of these tests with the multiple marker screen at 15 to 18 weeks.)
Some medical centers offer different ways of integrating the results of first and second trimester screenings, including the nuchal translucency screening, the blood tests for freeBeta-hCG and PAPP-a, and the multiple marker quadruple screen test. You can either find out the results of each test as you go along or wait until all the tests have been done for a more complete risk analysis. These integrated screenings are 85 to 95 percent accurate in detecting Down syndrome. You can discuss the pros and cons of each option with your caregiver or a genetic counselor.
Finally, a few very advanced medical centers include one more factor in the risk assessment: They look for the baby's nasal bone. If it's not there at all between 11 and 14 weeks, the baby is more likely to have Down syndrome. 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 of the nuchal fold and first trimester combined screening tests?
These screening tests 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 your baby's 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 11 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 not invasive and is no riskier than an ordinary ultrasound. And even if you decide not to have 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. This ultrasound 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 for sure whether your baby has normal chromosomes. In some cases they cause needless intervention and worry, and in other cases they're falsely reassuring.
Also, the nuchal fold test doesn't detect neural tube defects such as spina bifida and other abnormalities 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.