Nuchal Translucency Scan

What Is Nuchal Translucency?

Nuchal translucency refers to the sonographic appearance of fluid collection behind the neck of a fetus (underneath the skin) that is evident in the first trimester of pregnancy.

What Is a Nuchal Translucency Scan?

A nuchal translucency scan (NTS) is an ultrasound scan used to measure the nuchal translucency of a baby during the first trimester, in order to diagnose chromosomal abnormalities such as Down syndrome.

Who Should Undergo a Nuchal Translucency Scan?

A nuchal translucency scan is recommended for all pregnant women but is not mandatory. Women who suspect that their child may be born with a chromosomal abnormality for whatever reason, and women who are at risk of delivering a baby with a chromosomal abnormality due to their age or health conditions may consider undergoing a nuchal translucency scan.

When Should One Undergo a Nuchal Translucency Scan?

• A nuchal translucency scan is offered to all pregnant women in the first trimester along with a blood test.

• Between 11 and 14 weeks of pregnancy is the exact timeline for a scheduled nuchal translucency scan since this time period corresponds to the time when the base of the baby’s neck is transparent.

• The estimated last date a woman can undergo the NTS is at 13 weeks and 6 days of pregnancy.

Why Is a Nuchal Translucency Scan Done?

A nuchal translucency scan is done for the following reasons:

• To assess the risk of the baby being born with Down syndrome (trisomy 21).

• To assess the risk of the baby having rare chromosomal abnormalities like Patau syndrome (trisomy 13) or Edward’s syndrome (trisomy 18).

• To estimate the possibility of other chromosomal abnormalities in the baby.

• To screen for structural problems of the heart (congenital heart diseases), skeletal system, and abdominal wall, among others.

Who Performs Nuchal Translucency Scans?

• Trained technicians or personnel called sonographers are typically in charge of performing the nuchal translucency scan.

• Alternatively, a trained doctor, nurse, or midwife may perform the scan.

What Does the Nuchal Translucency Scan Procedure Involve?

The following steps are involved in the nuchal translucency scan procedure:

• Crown to Rump Measurement: The sonographer first measures the length of the baby from crown to rump to determine the approximate gestational age and whether or not the baby is the right size for that age.

• Sensor Positioning: The sonographer then takes an ultrasound scan of the abdomen. For this, they will first apply some gel over the abdomen of the mother, and then position the sensor or transducer over the area. The probe is gently run across the abdomen. The nuchal translucency of the baby should be visible on the monitor at this point. The procedure does not cause any pain to either the mother or the baby.

• Transvaginal Test: A lubricated ultrasound probe (small-sized probe) may be inserted into the vagina rather than placed over the abdomen in some mothers. The sonographer or doctor will determine which of these options is best, depending on factors like the time of pregnancy that the mother is undergoing the scan, and the body shape of the mother. The transvaginal ultrasound may cause minor discomfort, but is usually painless. The advantage of this method is the better detail and clarity provided in the images obtained since the probe is placed close to the uterus.

• Thickness Measurement: The thickness of the nuchal translucency is then measured on the screen using calipers.

How Are the Results of a Nuchal Translucency Scan Calculated and Evaluated?

• An ultrasound scan works through the use of high frequency sound waves.

• These sound waves are used to create images of the inside of the mother’s body.

• The specific image of interest to the doctor or technician in this case, is that of the back of the baby’s neck (specifically the base of the neck).

• The translucency (clear space) present at the back of the baby’s neck is measured and the data is inputted into a computer program along with details such as the age of the mother.

The variables that are used for the calculation of risk include:

• The age of the mother. The greater the age of the mother, the higher the chances of chromosomal abnormalities in the baby.

• The gestational age of the baby.

• The nuchal translucency measurement value.

• The results of the blood test were carried out along with the scan. This blood test usually measures the levels of the maternal hormone human chorionic gonadotropin (hCG) and plasma proteins. Abnormalities in either of these parameters are also potential indicators of chromosomal abnormalities in the baby.

• The approximate required thickness of nuchal translucency at a given gestational age is already pre-measured. The greater the thickness of the nuchal translucency at a given gestational age, the greater the chances of developing chromosomal abnormalities.

• The results will be expressed in terms of the chances of the baby having a chromosomal defect and will be in ratio form. Most tests use a cutoff of 1 in 250 or 1 in 300 to indicate normal or abnormal results. By this token, a result that falls anywhere in the vicinity of 1 in 300 (where the second number may be lower than 300) indicates a high risk. Values such as 1 in 1000 may therefore demonstrate a much lower risk.

Is the Nuchal Translucency Scan Accurate?

• The accuracy rate of a nuchal translucency scan differs depending on the scan itself and the concurrent tests.

• If the NTS is combined with a blood test to measure maternal serum free β-hCG (beta-human chorionic gonadotropin) and PAPP-A (pregnancy-associated plasma protein-A), the accuracy increases.

• Additional testing such as the examination of ductus venosus flow and tricuspid flow, as well as examination of the nasal bone may ensure that the detection rate is greatly improved, with a low possibility of false positive rates.

What Are the Prerequisites and Protocols Associated With Getting a Nuchal Translucency Scan Done?

• The gestation period of the baby should fall between 11 weeks and 14 weeks.

• The fetal crown-rump length should measure between 45 mm to 84 mm.

• Image magnification should be done such that the head and thorax of the fetus are the only visible parts on the screen or should fill the screen.

• Ensure that there is a clear distinction between fetal skin and amnion. The sonographer should cautiously assess and be able to distinguish between the two.

• The exact view required is the midsagittal view of the face. Even minor deviations can result in incorrect visualization and evaluation.

• The position of the fetus should be neutral (the head should be in line with the spine).

• The widest part of the translucency is to be measured.

• The calipers should be placed such that the inner border of the horizontal line is exactly on the line denoting the thickness of nuchal translucency.

• Ensure that multiple measurements are taken, so that the one that best fits all the prerequisites and protocol may be entered into the database and used to calculate the risks.

How Should One Prepare for a Nuchal Translucency Scan?

One need not prepare for a nuchal translucency scan. The scan is usually completed within 30 minutes. However, some of the instructions that may be given by the doctor include:

• Drink a few cups of water an hour before the scan, as a full bladder facilitates easier viewing of the uterus, and better reading of the images obtained.

• Wear loose clothing to make it convenient for the sonographer to access the lower abdomen.

Is a Nuchal Translucency Scan Safe for the Mother and Baby?

• The nuchal translucency scan is not an invasive test.

• It is also safe and causes no harm or pain to the mother or the baby.

• Nuchal translucency scans do not increase the risk of miscarriage.

What Should Be Done if the Results of a Nuchal Translucency Scan Are Positive for a Chromosomal Abnormality?

• It should be noted that nuchal translucency scans do not diagnose Down syndrome and other chromosomal abnormalities. The test can only predict the risk of abnormality.

• If the results returned indicate a high risk, parents may choose to proceed with other tests that provide definitive results. These tests may include amniocentesis and chorionic villus sampling procedures.

• Non-invasive prenatal testing (NIPT) or cell-free DNA testing (cfDNA) may be done either after obtaining a high-risk result from the NTS or as an alternative. NIPT is more accurate, but also more expensive than the NTS.

• Parents may also choose to wait until the second trimester to undergo a quad screen (a blood test) or a detailed ultrasound scan during weeks 18 to 20 of pregnancy.

• If they so choose, parents may consult with a practitioner or genetic counselor to determine the appropriate course of action following the test.

Conclusion:

A nuchal translucency scan is a good way to estimate the possibility of chromosomal abnormality in a baby. However, it should not be relied on as a sole diagnostic indicator. Parents need not worry about the risks to their babies purely based on NTS results but should proceed to undergo other, more definitive scans if they would like to. Some parents may choose not to undergo such scans, and that is absolutely fine too.