- A nuchal translucency (NT) ultrasound is a prenatal screening test typically done between 11 to 13 weeks of a woman’s pregnancy to help detect potential chromosomal abnormalities related to Down syndrome in the child(1).
- An average NT measurement is 2.18 millimeters (mm)(2). Higher measurements indicate a potentially higher risk for the child to develop abnormalities.
- Individuals working or aspiring to work as sonographers can attend online courses, such as those administered by the Fetal Medicine Foundation, to earn an NT scanning certificate(3).
What Is a Nuchal Translucency Test?
A nuchal translucency screening, also called NT screening, is a specialized routine ultrasound examination that pregnant women undergo at the end of their first trimester.
The test results of the procedure help obstetrics and gynecology doctors determine the baby’s statistical likelihood of developing a chromosomal abnormality.
Fetal nuchal translucency is the appearance of a subcutaneous (under the skin) fluid collection at the back of the fetus’ neck. This translucency typically occurs during the pregnancy’s late first and early second trimester.
Nuchal translucency is different from the nuchal fold, which is a normal skin fold at the back of the neck. Nuchal folds also appear during the second trimester.
NT screening focuses on the small, clear space at the back of the baby’s neck called the nuchal fold.
Uses of a Nuchal Translucency Test
Doctors use NT ultrasound for prenatal screening on pregnant women to detect chromosomal abnormalities associated with Down syndrome (trisomy 21). This condition is among the most prevalent genetic conditions affecting 1 in 700 babies in the United States annually(4).
Down syndrome is a condition wherein the child has an extra copy of chromosome 21. Children with this condition typically have a flat facial profile, upward slanting eyes, a tongue that sticks out, and small ears.
Additionally, almost half of all children with Down syndrome have congenital heart defects and need to be checked with an echocardiogram (heart ultrasound)(5).
The screening also determines the possibility of risk for Patau syndrome (trisomy 13) and Edwards syndrome (trisomy 18).
Patau syndrome, resulting from having three chromosome 13 copies, is associated with the brain, spinal cord, or cardiac defects, poorly developed eyes, a cleft lip, extra fingers or toes, or hypotonia (weak muscle tone)(6).
Meanwhile, individuals with Edwards syndrome have low birth weight, experience slow growth before birth, and have severe intellectual disability(7). Many children with this disease die before birth or within the first month.
Doctors typically perform the NT ultrasound on mothers who are between 11 and 13 weeks pregnant. This period is when the nuchal translucency, the clear tissue behind a developing baby’s neck, can be sufficiently measured.
An average NT measurement is around 2.18 millimeters (mm)(8). Indications of a higher or abnormal NT measurement during assessment present an increased risk of fetal abnormalities.
For example, abnormal NT levels during the first trimester can raise the child’s risk of developing trisomy 21 or Down syndrome(9).
Studies showed that up to 13% of fetuses with normal chromosomal presentation reflect a greater-than-average NT measurement of 2.5 millimeters(10).
However, the NT test does not provide a prenatal diagnosis of chromosomal abnormalities. Instead, the test assists the medical practitioner in determining the risk for such conditions and whether or not further testing is required.
Benefits and Risks
One benefit of NT screening is that it allows doctors and pregnant women to determine the probability of a child developing abnormalities later.
Thus, this procedure may help parents make objective decisions about what to do with their infant even before the child is born.
However, the nuchal translucency detection rate to determine whether the child has Down syndrome or other abnormalities is not 100% accurate. Performing NT screening alone can lead to false positives, which are relatively common with this procedure.
Thus, this screening is often combined with a blood test to give the physician added insight into the likelihood that a baby will be born with a genetic disorder.
Additionally, an NT test is a prenatal screening procedure, meaning it cannot diagnose any medical condition.
However, this combined first-trimester screening is also not a diagnostic test. This procedure cannot determine whether the baby has Down syndrome, Patau syndrome, or Edwards syndrome.
Instead, the screening shows the probability of whether the baby will have any of those syndromes. This probability, or chance, is based on maternal age, ultrasound information, and bloodwork(11).
These screening tests can inform the doctor and the mother whether the baby has an increased risk for chromosomal disorders or low risk for such conditions.
Certification Requirements
Requirements to earn a certificate in NT scanning may vary depending on the institution.
For example, the Fetal Medicine Foundation (FMF) requires attendees to complete the following to obtain an FMF certificate of competence for NT scanning(12):
- Attend the 11 to 13 weeks scan course, available online and free.
- Submit a logbook containing three images demonstrating nuchal translucency measurement.
Tests and Procedures
NT screening involves an ultrasound and maternal bloodwork. The ultrasound confirms the pregnancy’s progress.
In addition to detecting the fluid under the skin behind the baby’s neck, the ultrasound will also check the presence of the child’s nasal bone.
The test also takes a maternal blood sample to analyze the following chemicals found in the blood of all pregnant women:
- Pregnancy-associated plasma protein-A (PAPP-A)
- Free beta-human chorionic gonadotropin (hCG)
- Alpha-fetoprotein (AFP)
In pregnancies where the infant has Down syndrome, Patau syndrome, or Edward syndrome, there is typically extra fluid content behind the baby’s neck, or the hCG, PAPP-A, and AFP results are lower or higher than the average.
Additionally, a baby’s nasal bone may be absent in pregnancies with chromosomal abnormalities. However, most babies with this finding are normal(13).
Combining the mother’s age-related risk with the NT measurement, bloodwork results, and nasal bone data provides doctors with one risk figure for Down syndrome and another risk figure for trisomy 13 or trisomy 18.
Preparation
When preparing the mother and child for NT screening, sonographers must follow the following protocols when performing the measurement(14):
- The gestation period must be between 11 and 13 weeks and six days.
- The fetal crown-rump length should measure between 45mm and 84mm.
- The image should be sufficiently magnified so that the fetal head and thorax occupy the entire screen.
- Sonographers should present a midsagittal view of the face.
This view is obtained by presenting the nose tip and palate anteriorly (near the front), the diencephalon (the brain part containing the hypothalamus) in the center, and the nuchal membrane posteriorly (near the rear).
The sonographer should carefully take the image because slight deviations from the exact midline plane can cause non-visualization of the nose tip and visibility of the maxilla (bones forming the dominant portion of the face).
- The fetus should be positioned neutrally, where the head is aligned with the spine. Otherwise, a hyperextended fetal neck can falsely increase the measurement, or a flexed neck can falsely decrease the measurement.
- Sonographers should distinguish between the amnion (outer fluid-filled membrane) and fetal skin.
- Sonographers should always measure the widest part of nuchal translucency.
- Measurements are taken with the inner border of the calipers’ horizontal line placed on the line defining the nuchal translucency thickness.
The calipers’ crossbar should be hardly visible since it should merge with the white borderline and not in the nuchal fluid.
- Turning down the “gain” when magnifying the image during pre- or post-freeze zoom is essential to avoid placing the caliper on the blurry edge of the line. Doing so can cause an underestimation of the nuchal measurement.
- During the scan, the sonographer should take more than one measurement. The full scan satisfying all the above criteria is recorded in the database.
- The umbilical cord may be around the fetal neck in about 5% of cases, which can produce a falsely increased NT. The NT measurements above and below the cord are different in such cases.
Thus, using the average of the two measurements is more appropriate when calculating the risk.
Treatment and Prognosis
As the second trimester approaches, the region with nuchal translucency can either regress, evolve into a complication or congenital disability, or continue to have a normal outcome.
Although NT screening is not a procedure to treat such disabilities, doctors can refer to its results to recommend or prescribe treatments should a disease be confirmed in later examinations.
The region is likely to regress if it is chromosomally normal, resulting in fetuses having a normal outcome. However, a spontaneous regression does not necessarily translate to having a normal karyotype(15).
A karyotype is an individual’s complete set of chromosomes.
However, nuchal translucency can also evolve into nuchal edema or cystic hygroma.
Cystic hygroma appears as a thin-walled, sac-like structure commonly developing in an infant’s head or neck area. Meanwhile, fetal nuchal edema is characterized by a swollen fetal neck region and can be considered a less severe cystic hygroma.
For normal outcomes, the change in likelihood is proportional to the following NT values(16):
- At 3.5mm to 4.4 millimeters (mm), there is a 70% chance of becoming normal
- From 4.5mm to 5.4mm, the likelihood of becoming normal is 50%
- At 5.5mm to 6.4mm, the chance is 30%
- For values greater than 6mm, the likelihood of being normal is 15%
Sample Cases
The following sample cases show various NT screening measurements, follow-up examination results, and the status of the infant at birth.
Case 1
This case features a 33-year-old pregnant woman who conceived by intra cytoplasmic sperm injection (ICSI) and underwent a routine screening test(17).
She had no remarkable obstetric and medical histories. However, the routine first trimester ultrasound results at 11 weeks and five days gestation showed a significantly increased NT. The normal range of NT for this age is 1mm to 2.8 mm.
Later examination revealed that the NT fluid was enveloping the fetus and not only limited to the neck. The researchers reported the follow-up results, such as prenatal ultrasound, echocardiography, and amniocentesis, to be normal.
The woman gave birth to a healthy baby boy by elective cesarean section at term. The child weighed 3,500 grams (g) at 37 weeks of pregnancy.
Follow-up after birth for up to two years revealed the child did not have any developmental delay.
Amniocentesis is a procedure wherein a small sample from the amniotic fluid (clear, yellowish liquid surrounding the fetus) is taken for testing.
Amniocentesis can also help determine if the child has Turner syndrome(18). This disease affects only females and can cause developmental issues, such as heart defects, short height, and failure of ovaries to develop.
Case 2
This case involves a 35-year-old woman referred for a routine screening test during her pregnancy(19). Her medical and obstetric history revealed primary infertility for seven years, polycystic ovaries, and mild endometriosis.
Polycystic ovary syndrome (PCOS) is a condition where tiny fluid-filled sacs (cysts) develop in the ovaries due to an abnormal amount of androgens (male sex hormones in women).
Endometriosis is a painful disorder where the tissue usually lining inside the uterus grows outside the uterus.
The NT was measured at 3mm during the first trimester screening at 13 weeks. The normal NT range for that age is 1.6mm to 2.4mm.
Moreover, nuchal skin fold (NF) measurements and follow-up prenatal ultrasound results were normal. A triple test performed on the woman showed a high risk of the child getting trisomy 21.
The patient was referred for chorionic villus sampling (CVS) or amniocentesis. Chorionic villi are tiny, finger-like projections of placental tissue with the same genetic composition as the fetus.
The amniocentesis findings were normal, and a healthy baby girl was born weighing 3,600g after a full-term pregnancy. During the first and second years of life, follow-up examinations showed no developmental delay.
Case 3
The individual in this scenario is a 30-year-old patient with a twin pregnancy at 12 weeks and one day of gestation(20). The second fetus was diagnosed with an NT of 3.5mm. The normal NT range for this age is 1.2mm to 3 mm.
Follow-up prenatal ultrasound revealed normal results. However, the patient refused amniocentesis and echocardiography.
Two healthy babies, a boy and a girl, were born at 37 weeks of pregnancy. The newborns’ weights were 2,400g and 2,900g, respectively. Follow-up examinations during their first and second years revealed no developmental delay.
Expected Results
When used as a standalone assessment tool to detect the babies’ risk for chromosomal abnormalities, NT ultrasound screening has an accuracy rate of 70% to 75%, with a five-percent false-positive rate(21).
Many doctors, such as obstetricians and gynecologists, recommend pairing NT ultrasound screening with the quad screen (integrated screening) or non-invasive prenatal testing (NIPT).
This combination helps improve the accuracy of identifying potential chromosomal abnormalities risk factors to 83% to 92%(22).
History
In 1992, researchers led by K.H. Nicolaides (cited in some sources as Nicolaides K.H.) analyzed the neck fullness among neonates (newborn children) with Down syndrome and the relation of nuchal edema or cystic hygroma in fetuses with chromosomal abnormalities during the second trimester(23).
The researchers suggested that an ultrasound scan showing an increase in nuchal thickness may indicate fetal aneuploidy (the presence or absence of one or more chromosomes).
The same authors observed 827 fetuses in pregnant women who underwent transabdominal ultrasound at a gestational age of 10 to 14 weeks(24).
The researchers measured the maximum thickness of the fluid behind the neck of the women’s fetuses and coined the term “nuchal translucency” to refer to this liquid.
Results showed that fetuses with a nuchal translucency of 3 to 8mm had a 35% incidence rate (18 cases) of developing chromosomal abnormalities. On the other hand, only 1% of the fetuses with smaller measurements had chromosomal defects.
Thus, the researchers suggested that increased nuchal translucency may be associated with an increased risk for fetal abnormalities.
According to the Terminologia Anatomica, “nucha” is the official Latin term that refers to the nape or back of the neck.
FAQs
1. What is a normal nuchal translucency measurement?
The normal NT measurement usually depends on the gestation period. One study analyzing the normal reference range of fetal NT thickness showed that at 12 weeks, the median NT thickness was 1.2mm(25).
Another study showed that the normal NT range for an infant at 12 weeks and one day of gestation is around 1.2mm to 3 mm(26).
2. How accurate is the NT scan for Down syndrome?
The first trimester NT screening detection rate can be as high as 96% for pregnancies which the baby has Down syndrome(27). The detection rate can be higher for pregnancies with trisomy 13 or 18.
However, an NT ultrasound performed without bloodwork may only have a 70% detection rate. Thus, a first trimester screening may work better when NT ultrasound is combined with other tests.
- What to Expect from a Nuchal Translucency (NT) Ultrasound
https://www.cwcare.net/news/what-expect-nuchal-translucency-nt-ultrasound - Ibid.
- Nuchal Translucency Scan
https://fetalmedicine.org/fmf-certification-2/nuchal-translucency-scan - What to Expect from a Nuchal Translucency (NT) Ultrasound
https://www.cwcare.net/news/what-expect-nuchal-translucency-nt-ultrasound - Down Syndrome
https://kidshealth.org/en/parents/down-syndrome.html - Trisomy 13
https://medlineplus.gov/genetics/condition/trisomy-13/ - Trisomy 18
https://medlineplus.gov/genetics/condition/trisomy-18/ - What to Expect from a Nuchal Translucency (NT) Ultrasound
https://www.cwcare.net/news/what-expect-nuchal-translucency-nt-ultrasound - Nuchal Translucency Measurement
https://www.sciencedirect.com/topics/medicine-and-dentistry/nuchal-translucency-measurement - What to Expect from a Nuchal Translucency (NT) Ultrasound
https://www.cwcare.net/news/what-expect-nuchal-translucency-nt-ultrasound - Combined First-Trimester Nuchal Translucency Screening
https://www.hopkinsmedicine.org/gynecology_obstetrics/specialty_areas/maternal_fetal_medicine/services/fetal_diagnostic_procedures/first_trimester_nuchal_translucency_screening.html - Nuchal Translucency Scan
https://fetalmedicine.org/fmf-certification-2/nuchal-translucency-scan - Combined First-Trimester Nuchal Translucency Screening
https://www.hopkinsmedicine.org/gynecology_obstetrics/specialty_areas/maternal_fetal_medicine/services/fetal_diagnostic_procedures/first_trimester_nuchal_translucency_screening.html - Nuchal Translucency Scan
https://fetalmedicine.org/fmf-certification-2/nuchal-translucency-scan - Nuchal Translucency
https://radiopaedia.org/articles/nuchal-translucency-1?lang=us - Ibid.
- Chromosomally and Anatomically Normal Fetuses With Increased First Trimester Nuchal Translucency Conceived by ICSI
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4457970/ - Turner Syndrome: Diagnosis & Treatment
https://www.mayoclinic.org/diseases-conditions/turner-syndrome/diagnosis-treatment/drc-20360783 - Chromosomally and Anatomically Normal Fetuses With Increased First Trimester Nuchal Translucency Conceived by ICSI
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4457970/ - Ibid.
- What to Expect from a Nuchal Translucency (NT) Ultrasound
https://www.cwcare.net/news/what-expect-nuchal-translucency-nt-ultrasound - Ibid.
- A Historical and Practical Review of First Trimester Aneuploidy Screening
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6596981/ - Fetal Nuchal Translucency: Ultrasound Screening for Chromosomal Defects in First Trimester of Pregnancy
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1882788/pdf/bmj00067-0019.pdf - Normal Reference Range of Fetal Nuchal Translucency Thickness in Pregnant Women in the First Trimester, One Center Study
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4746871/ - Chromosomally and Anatomically Normal Fetuses With Increased First Trimester Nuchal Translucency Conceived by ICSI
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4457970/ - Combined First-Trimester Nuchal Translucency Screening
https://www.hopkinsmedicine.org/gynecology_obstetrics/specialty_areas/maternal_fetal_medicine/services/fetal_diagnostic_procedures/first_trimester_nuchal_translucency_screening.html