Prenatal screenings and tests provide useful information about the baby’s health before the little one enters into the world. Screening tests are usually done during the first and second trimesters to determine if there are any health risks to your baby.
Majority of the tests performed during the early stage are noninvasive procedures. One such screening is nuchal translucency scan or NT scan. In this MomJunction post, we tell you when this procedure is done, why it is done, and what the process is.
What Is A Nuchal Translucency Scan?
Nuchal translucency refers to the sonographic image showing the fluid accumulation behind the fetus’ neck.
The nuchal translucency or NT scan is an ultrasound test to measure the thickness of this fluid accumulated in the space behind the fetus’s neck tissue. Increased fluid could indicate a risk of abnormality (such as Down Syndrome or other genetic disorder or birth defect) to the baby.
This test only lets you know if the baby has a chance of any risks, and not about the risk or the abnormality. Also, it scans for the risks of congenital heart problems. It is a part of the first-trimester screening and is done alongside the mom-to-be’s blood work (1).
When Is Nuchal Translucency Screening Done?
The ideal time for this screening is between 10 and 14 weeks, and the fetal crown-rump length (CRL) should be in the range of 45mm and 84mm.
During this time, the fetal lymphatic system is at its developing stage, and the placenta’s peripheral resistance is high, which means it is the right time to detect if there is any risk of abnormality.
Before ten weeks, the fetus is too small to check for NT measure. And after 14 weeks, the additional fluid is drained, making it difficult to determine the nuchal translucency measurement (3).
How Is The NT Scan Done And How Much Time Does The Procedure Take?
The scan is usually transabdominal, but in some cases, it could be transvaginal. The sonographer will put a gel on your belly and use a transducer over it. The professional will observe the black and white image of the fetus on the monitor to measure the fluid. The black part on the image is the fluid, while the white line is the skin.
This non-invasive procedure causes no known harm to the mother or the baby. During the scan, the image magnification focuses on the head of the fetus and thorax (4). It takes approximately 30 minutes for the scan to get completed.
The scan also helps to see if the fetus has a nasal bone. To detect the risk of chromosome abnormalities (Down syndrome, trisomy 13 or trisomy 18), the results of NT measurement, blood work (pregnancy-associated plasma protein-A (PAPP-A) and hCG), and nasal bone data are combined and analyzed. This combined tests can improve the detection rate. In some cases, when the results of PAPP-A and hCG are lower or higher than average, there could be risks of trisomy 13, trisomy 18, or Down Syndrome (5).
For instance, if the combined test results show a chance of 1 in 500, it means the likelihood of the baby having the abnormality is one in 500 cases, other diagnostic tests are recommended to confirm the risk.
What Is The Normal Translucency Measurement?
NT below 3mm is considered normal between the gestation weeks 11 and 14, and CRL between 45 and 84mm. Ideally, the thickness of nuchal translucency increases with the CRL.
To be more precise, the normal NT ranges from 1.2 to 2.1mm when the CRL is 45mm. And when the CRL is 84mm, the normal NT range is from 1.9 to 2.7mm. But if it is between 3 and 3.5mm, then it is considered high (6) (7).
Note: The NT measure is then analyzed per the gestational age and maternal age to know the risks of abnormality.
How Accurate Is The Nuchal Translucency Scan?
The NT scan is only a screening test, which shows whether the baby has any chance of congenital disabilities. The accuracy of the results can be determined when the following diagnostic tests are done to confirm any suspicions of a birth defect.
If the result of the screening test is positive, it means the baby could have a defect. But the diagnostic test may show that the baby is healthy with no problem. In such cases, the NT result is called a false-positive. In another case, the screening might show negative, but the actual diagnostic test could be positive, indicating that the baby has a defect. This is called a false-negative. Therefore, the NT scan cannot be considered 100% accurate (8).
What Happens If Nuchal Translucency Scan Indicates A Problem?
In case the NT scan indicates a risk of any abnormality (be it trisomy 13 or 18, or Down syndrome), you should know that it doesn’t necessarily mean that your baby actually has the abnormality or is definitely at risk of developing it. However, in case the measurement is high, a genetic counselor will explain the next course of action to you.
If the NT scan indicates a risk, amniocentesis and chorionic villus samples are the diagnostic tests recommended to further detect if there is any chromosomal abnormality. The accuracy of these tests is higher than 99% (5).
If any of these tests show a high risk of birth defects, the doctor will assign a counselor or a special professional team (doctors, nurses, and midwives) to explain the findings of the test and other necessary aspects to help you make decisions.
Based on the risks and other crucial details, you (along with your partner and family) may decide to either wait for the second-trimester test results or go for a second opinion or terminate the pregnancy after careful thought.
Can Nuchal Translucency Determine Fetal Gender?
NT scan is done to determine if the fetus carries any chance of abnormality risk. In some cases, it is possible to determine fetal gender through the first-trimester scan. But the results might not be accurate. The chances of finding out the fetus’ gender are better when the second-trimester scans are done.
Is It Compulsory To Go For NT Scan?
No. You may opt for the NT screening if you want to know more about your baby’s development. Diagnosing any condition at an early stage of pregnancy can provide you with options.
Nuchal translucency scan is an optional screening test that is suggested to pregnant women during the first trimester. The test harms neither you nor your baby as it is a non-invasive ultrasound scan. However, if you are unsure about it, talk to your doctor and make a call.
Did you take NT scan when you were pregnant? You may share your experiences in the comment section below.
2. Nuchal translucency screening test; Michigan Medicine
3. A. Berger; What is fetal nuchal translucency; the BMJ (1999)
4. Nuchal translucency scan; The Fetal Medicine Foundation
5. Maternal serum screening, nuchal translucency scan and nasal bone sonogram; Gynecology & Obstetrics; John Hopkins Medicine
6. When a normal karyotype accompanies increased NT; Society for Maternal Fetal Medicine
7. R. D. Domenico at al.; Increased nuchal translucency in normal karyotype fetuses; Journal of Prenatal Medicine (2011)
8. First-trimester screening for birth defects; Michigan Medicine