What is the nuchal translucency scan?
The nuchal translucency scan (also called the NT or nuchal fold scan) is a prenatal screening test that can help your doctor assess your baby’s risk of having Down syndrome (DS) and some other chromosomal abnormalities as well as major congenital heart problems.
The NT test uses ultrasound to measure the clear (translucent) space in the tissue at the back of your developing baby’s neck. Many 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.
The NT scan 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.) Like other screening tests, an NT scan won’t give you a definitive diagnosis for a non-structural abnormality, like a chromosomal or a genetic abnormality. But it can assess your baby’s risk/ probability for certain problems and help you decide whether you want to have definitive testing, i.e, Chorionic Villous Sampling (CVS) or Amniocentesis to find out whether your baby is actually affected or not.
Ultrasound technicians (sonographers) and doctors performing the NT Scan need special training and high-resolution ultrasound equipment to perform it correctly. They must be certified by the Fetal Medicine Foundation in London, the organization that sets the international standards and provides the software that enables a doctor to evaluate your baby’s risk.
The NT scan requires special training and equipment and therefore it is not yet available everywhere. But the test is being done more widely as more medical personnel are getting trained and certified.
How is the NT scan done?
It is usually an abdominal Scan. In some situations if the visualization is difficult or the baby is in a difficult position, a Transvaginal approach (internal scan) can be used. The sonologist first confirms your baby’s gestational age by measuring the fetal ‘Crown to Rump Length’ (CRL) to see if the size is correct for the stage of pregnancy. Then the baby’s neck thickness (Nuchal Translucency, i.e, NT) is measured. The fetal anatomy is checked thoroughly from ‘head-to-toe’ to confirm the presence or absence of major fetal structural problems. Additional ‘Markers” like the presence or absence of the nasal bone, abnormal blood flow across the heart valve (Tricuspid Valve) or measurement of the facial angle is used to improvise the risk assessment.
How are the results computed?
Your baby’s chances of having a chromosomal abnormality are determined by your age, the stage of pregnancy, the NT measurement, the baby’s heart rate and presence or absence of additional “markers”. If you have done the First Trimester Double Marker Blood Test then, these values are also included to compute your baby’s individual or personalized risk of having a chromosomal abnormality. You may get the results right away for the nuchal translucency test, or you may have to wait few days depending upon the inclusion of blood tests.
What do the results mean?
You’ll get your results in the form of a ratio that expresses your baby’s chances of having a chromosomal defect.
For example, you may be told that your baby’s chances of having Down Syndrome are 1 in 30 or 1 in 4,000. A risk of 1 in 30 means that for every 30 women with this same result, one baby will have Down Syndrome and 29 will not. A risk of 1 in 4,000 means that for every 4,000 women with the same result, one baby will have it and 3,999 will not. The higher the second number, the lower is the risk.
Remember that a ‘normal’ screening result isn’t a guarantee that your baby has normal chromosomes, but it does suggest that a problem is less likely. Likewise, an abnormal screening result doesn’t mean that your baby definitely has a chromosomal problem — just that its more likely to have one. In fact, most screen-positive babies turn out not to have a problem.
What if the NT is increased and the chromosomes are normal?
If your baby’s nuchal fold measurement is increased for the gestational age, and even if the chromosomes are normal, it is still considered to be at increased risk for major congenital heart disease, so you’ll be offered a special ultrasound called a ‘Fetal Echocardiogram’. If your baby has a heart defect, it’ll need to be monitored with regular ultrasounds and delivered at a medical center equipped to handle this condition.
You will be given an opportunity to discuss this in more detail with the Paediatric Cardiologist.
How accurate is this test?
How reliable a test is can be seen in its detection rate. The detection rate tells you how good the test is at identifying babies that are truly affected as being at high risk.
The NT scan alone will detect about 70 to 80 percent of babies with Down syndrome. The detection rate for the first-trimester combined screening (the NT scan plus a first-trimester blood test) ranges from 90 to 95 %.
(please see the information leaflet for combined screening for details of the combined test)
This means that if you’re carrying a baby with DS, there’s a 90 % chance that the Combined screening test will detect the abnormality and give you what’s called a ‘screen-positive’ result indicating that further testing is recommended. It also means there’s a 5 to 10 % chance that the tests will miss the DS and give you a ‘screen-negative’ result.
Screening tests aren’t perfect. They don’t detect all cases of Down Syndrome, so they may identify you as being at ‘low risk’ when your baby really does have Down Syndrome. This is called a ‘false negative’ result and it might lead you to decide against diagnostic testing that would have revealed a problem.
Conversely, screening may identify you as being at ‘high risk’ when your baby is fine (a ‘false positive’). This may cause you to undergo further testing and to worry about your baby’s well-being unnecessarily, when infact, the results come back ‘normal’.
What if the results indicate that my baby is at ‘high risk’ for a problem?
With the help of our Specialists, your Doctor and/ or our Genetic and Emotional counselors, 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. We encourage individual parents must decide for themselves what risks are acceptable in their situation.
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 detailed ultrasound at 18 to 23 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/ structural defects, such as heart defects, spina bifida etc. This is called a Genetic Sonogram which will re-assess your risks.