Have you heard about nuchal translucency measurement?

Nuchal translucency is an ultrasound performed during the first trimester of pregnancy. The goal is to determine your baby's risk of having Down syndrome and certain other chromosomal (chromosomal) abnormalities. A nuchal translucency ultrasound is usually done along with a blood test. 

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• 1. What is nuchal translucency measurement?
• 2. When is nuchal translucency measurement performed?
• 3. How is nuchal translucency measured?
• 4. What is the nuchal translucency measurement to screen for?
• 5. When is the nuchal translucency measurement result abnormal?
• 6. What are the benefits of the first trimester screening test?
• 7. Accuracy and limitations of the first trimester screening test?

1. What is nuchal translucency measurement?

Nuchal translucency is the use of ultrasound to assess the risk of Down syndrome and Trisomy 18. Ultrasound can also detect other major congenital heart defects. This test is recommended for all pregnant women. Usually, the nuchal translucency measurement will be combined with a blood test.

Nuchal translucency means measuring the space in the tissue at the back of your baby's neck. Infants with abnormalities tend to accumulate more fluid at the nape of the neck during the first trimester. 

This test uses transvaginal or transvaginal ultrasound. Therefore, the test is not invasive or harmful to mother and fetus. However, the test results can only suspect that the child has a high probability of having Down syndrome or Trisomy 18, but cannot confirm that the child will definitely have these diseases.

2. When is nuchal translucency measurement performed?

This test will need to be done when you are between 11 and 14 weeks pregnant, as this is when the back of your baby's nape is still transparent. The last day you can take this test is when you are 13 weeks and 6 days pregnant. Or when the baby's rump length (CRL) is between 45 mm and 84 mm.

Nuchal translucency measurement is done when the mother is 11 to 14 weeks pregnant

Nuchal translucency is not performed before 11 weeks because the baby is still too small. Also, it's too early to incorporate blood testing in the first trimester.

Measuring after 14 weeks can be difficult, as excess fluid can be absorbed by the baby's developing lymphatic system. This makes the results inaccurate.

3. How is nuchal translucency measured?

Nuchal translucency measurement is usually done through the abdomen with normal ultrasound equipment. The doctor will put some gel on your belly. A hand-held device (transducer) will then be used and slid over the mother's abdomen. The test causes no pain or harm to the mother and baby. However, you may experience some discomfort when the sonographer presses the transducer into your abdomen. 

In some cases, it will be difficult to see the baby's morphology during abdominal ultrasound, for example, the abdominal wall is thick, fatty or the mother's uterus is tilted back. In this case, your doctor may recommend a transvaginal ultrasound. Don't worry because transvaginal ultrasound is still safe for both mother and baby.

Transvaginal ultrasound

Your doctor will first need to determine the exact number of gestational ages possible. The doctor will also use ultrasound to measure the length of the baby from the top of the head to the bottom of the spine. This length will estimate the gestational age of the baby. Once the fetal age has been checked, the ultrasound probe continues to measure the width of the fluid at the back of the baby's neck. Your baby's skin will appear as a white line and the fluid under the skin will be black. This test can take up to an hour.

In addition to measuring nuchal translucency, you will need to do additional blood tests:

For the blood test, a blood sample is taken from a vein in the mother's arm. Then the blood sample is sent to a laboratory for analysis of 2 indicators, B-HCG and PAPP-A. This is also known as the Double test. Nuchal translucency and double test are screening tests for the first trimester of pregnancy. These tests carry no risk of miscarriage and do not cause other pregnancy complications.

4. What is the nuchal translucency measurement to screen for?

4.1 Down syndrome (trisomy 21)

Down syndrome is an inherited condition that causes intellectual disability, congenital heart problems, and problems with facial features. This abnormality is due to the fact that instead of having 2 chromosomes 21, the baby has 3.

4.2 Edwards syndrome (trisomy 18):

This is a chromosomal abnormality characterized by having three chromosomes 18 instead of just two 18. Trisomy 18 often causes severe malformations and intellectual disability. Most babies with trisomy 18 die before birth or within the first year of life. 

4.3 Other malformations

A first-trimester ultrasound may also reveal some major heart or bone defects.

5. When is the nuchal translucency measurement result abnormal?

A nuchal translucency of less than 3.5 mm is considered normal when the baby measures between 11 and 13 weeks and 6 days or when the rump length is between 45 mm and 84 mm.

The images below show different levels of nuchal translucency:

Nuchal translucency 1.3 mm within normal range

Nuchal translucency is 2.9 mm

A baby with a nuchal translucency of 2.9mm is also within the limits of the normal range. However, if the measurement is from 2.5 to 3.5mm, then 1 out of 10 children will probably have 1 child with Down syndrome.

As nuchal translucency increases, the likelihood of Down syndrome and Trisomy 18 also increases.

Nuchal translucency is 6 mm

Babies with a nuchal translucency of 6mm have a high chance of having Down syndrome, as well as other chromosomal abnormalities and heart problems. It is very rare for an infant to have this much fluid.

However, even if nuchal translucency is within normal limits, blood test results and other factors should be considered, such as maternal age, personal history of Down syndrome or Trisomy 18 The doctor will record the results of the nuchal translucency measurement, blood test results and other factors into the database to provide an accurate estimate of the likelihood of Down syndrome.

6. What are the benefits of the first trimester screening test?

First trimester screening is done earlier than most other prenatal screening tests. So you will have results early in pregnancy. This gives the mother more time to make decisions about further diagnostic tests.

The baby is diagnosed with a genetic disease. Mothers will also have more time to prepare for the possibility of caring for a child that requires a lot of special care.

In addition, the nuchal translucency screening test and blood test only predict the likelihood of a child having a chromosomal disorder, but cannot confirm it completely. When the results return high-risk, the doctor will advise the mother to do additional tests of placenta biopsy or amniocentesis to make an accurate diagnosis. However, these two types of tests are invasive. So there will be some risk factor for miscarriage. The doctor will also carefully advise the mother about these tests when necessary. You can completely agree to have in-depth testing or not.

>> See more about important information mothers need to know during pregnancy ultrasound to easily update the health status of mother and baby. 

7. Accuracy and limitations of the first trimester screening test?

First-trimester screening correctly identifies about 85% of young pregnant women with Down syndrome. Of these, about 5% of women have a false-positive result, meaning the test is positive but the baby doesn't actually have Down's syndrome. For Trisomy 18, the detection rate is about 90% with a false positive rate of 2%.

>> Pregnancy in old age , have you learned about the potential benefits and risks?

Nuchal translucency testing is recommended for all pregnant women. This helps to detect fetal abnormalities early in the first 3 months. With early results, you will have more peace of mind that your pregnancy is healthy and normal. This will also give you more time to choose and make the best decisions for both you and your baby.

Doctor Nguyen Trung Nghia