Introduction:
Nuchal translucency is the sonographic appearance of the fluid collected under the fetus's neck. It is measured during the late first trimester or early second trimester. This nuchal translucency test assesses the risk of chromosomal abnormalities or cardiac defects and genetic syndromes.
Why Is the Nuchal Translucency Important?
The risk of chromosomal abnormalities is greater when the mother is older. Anyone can have a chromosomal abnormality. So screening is essential to assess the normal health of the baby. Increased translucency may be due to dilated lymphatic channels and is considered to be a fetal abnormality. This detects about 80 % of fetal trisomies (presence of an extra copy of chromosomes leading to developmental abnormalities) and major anomalies. A nuchal translucency scan or the NT scan detects the risk of any chromosomal abnormalities or genetic and cardiac defects in your baby.
When Is the Nuchal Translucency Scan Done?
A nuchal translucency scan or the NT scan is performed during the late first trimester or the early second trimester. More precisely, between 11 weeks three days to 13 weeks six days.
What Is the Normal Value of Nuchal Translucency?
Nuchal translucency value between 2.5 mm and 3 mm is considered to be normal. The values of more than 3 mm are associated with developmental and genetic abnormalities.
What Are the Complications Associated With Increased Nuchal Translucency?
Thickening of nuchal translucency or increase in the nuchal translucency value is associated with numerous abnormalities or health conditions. They are categorized into two:
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Aneuploidy defects.
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Non-aneuploidy defects.
Before getting into the complications, you should first know what aneuploidy is!
What Is Aneuploidy?
Aneuploidy is the presence of an abnormal number of chromosomes in the cell. They include:
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Down Syndrome or Trisomy 21: It is an extra copy of chromosome 21.
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Turner Syndrome: This affects only the female baby where only one X chromosome is present.
What Are Non-aneuploidy Defects?
Complications caused other than the abnormality in the number of chromosomes is the non-aneuploidy defects. They include:
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Miscarriage or Fetal Demise: The risk of fetal demise is directly proportional to the increase in the value of nuchal translucency.
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Congenital Heart Diseases: Septal defects are the common ones.
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Nooman’s Syndrome: It is a genetic disorder associated with heart diseases, facial deformities, bleeding disorders, etc. It has a clear association with increased nuchal translucencies.
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Congenital Diaphragmatic Herniation: Diaphragm is a thin sheet of muscle separating the chest from the abdomen. A congenital diaphragmatic herniation is the presence of a hole in the diaphragm muscle.
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Omphalocele Syndrome: Defects in the abdominal or belly wall.
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Skeletal Dysplasia: It is a group of disorders associated with bone and cartilage formation.
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Smith- Lemli-Opitz Syndrome: This is a developmental syndrome characterized by microcephaly, intellectual or learning disabilities, and behavioral problems.
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Intrauterine Infections: Infections with parvovirus B 19 result in fetal anemia and fetal myocardial infarction. It is the only pathogen-specific to increased nuchal translucency.
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VACTERL Associations: It affects many body parts. VACTERL stands for Vertebral defects, Anal atresia, Cardiac defects, Tracheoesophageal fistula, Renal abnormalities, and Limb abnormalities.
What Is a Combined First Trimester Scan?
Sometimes the nuchal translucency scan can be combined with the blood tests called the combined first-trimester scan CFTS. This combination of blood tests and ultrasound gives more accurate results. This test interprets:
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Whether the pregnancy is in the right place and rules out ectopic pregnancy.
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Fetal cardiac activity.
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Multiparity (whether you are carrying more than one baby).
Are There Any Alternatives for a Nuchal Translucency Scan?
Yes, non-invasive prenatal testing (NIPT), also called cell-free DNA (cfDNA), is an alternate test for ruling out chromosomal abnormalities. It is more accurate than the nuchal translucency scan.
How Is the Nuchal Translucency Scan Done?
The nuchal translucency scan is done abdominally (transabdominal) or vaginally (transvaginally).
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If the transabdominal scan is done, you will be instructed to drink a few cups of water to make your bladder full. This makes the inside of the uterus more visible. The sonographer applies a lubricating gel, and the ultrasound probe is moved across the abdomen. This method does not hurt the mother.
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In case your doctor performs a transvaginal scan, a lubricated probe is inserted into the vagina. This gives more detailed information as it is close to the uterus. This may cause discomfort in some patients, but it is not painful.
Does the Ultrasound Cause Any Harm?
Many mothers have a question about whether the ultrasounds are safe or not. Generally, ultrasounds are harmless. They do not cause any harm to the mother and the baby. There is no association between miscarriages due to ultrasound.
How Is the Nuchal Translucency Measured?
The sonographer usually performs the nuchal translucency scan. It is a technique-sensitive procedure. The nuchal translucency scan is measured on a sagittal plane (dividing the body into right and left) through the neck of the fetus.
What Is the Protocol to Measure Nuchal Translucency?
1. The fetus should be in a midsagittal plane(a vertical plane dividing the body into right and left). The vertebral column faces the bottom of the screen.
2. In order to check for the correct midsagittal position, the following structures should be visible.
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Two tiny echogenic lines.
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Tip of the nose.
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Nasal bone.
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Hard palate.
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Diencephalon.
3. Proper magnification is required. The head and the upper thorax of the fetus are only included in the image.
4. The head of the fetus should not be flexed or extended.
5. The fetus should be free-floating from the uterine wall. The amniotic fluid should be visible between the back of the fetus and the uterus.
6. The widest part of the translucency should be measured.
How Do We Interpret the Results?
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Values lesser than 2.2 to 2.8 mm are not associated with high risk. However, it should be correlated with maternal age, gestational age, and crown-rump length(CRL).
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Lesser than 2 mm → 1 % risk.
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3.4 mm → 7 % risk.
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3.5 mm to 4.4 mm → 20 %risk
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5.5 mm to 6.4 mm → 50 % risk.
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More than 8.5 mm → 75 % risk.
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Combined first-trimester screening is required when the crown-rump length is between 45 mm to 85 mm.
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The translucent region should not be septated.
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The amount of thickness is directly related to the incidence of chromosomal abnormalities.
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The nuchal translucency value for down syndrome is less than 4.5 mm.
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Trisomy 13 or 18 is 4.5 mm to 4.8 mm.
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Turner’s syndrome is greater than 8.5 mm.
The nuchal translucency can be correlated with serum markers such as:
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Maternal B Human Chorionic Gonadotropin Hormone hCG: Maternal hormone responsible for the pregnancy and initiates the placenta formation.
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Alpha-Fetoprotein (AFP): It is a protein secreted by the fetus present in the blood and amniotic fluid, which helps in the diagnosis of genetic abnormalities.
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Pregnancy-Associated Plasma Protein (PAPP): It is key for insulin and essential for normal fetal growth and development.
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Estriol: It is one of the estrogen hormones present in the blood during pregnancy. It helps and prepares the body during pregnancy and labor.
Conclusion:
Nuchal translucency detects the risk of chromosomal abnormalities in your baby during 11 to 13 weeks. The early detection thus prevents various health conditions in the fetus. Regular prenatal visits and creating an awareness regarding the importance of prenatal tests among the parents are important. If the abnormal nuchal translucency values and screening tests show a high risk, parents need not panic. They can do amniocentesis or chorionic villi sampling and fetal echocardiography. As there are advances in the medical field, various diseases can be prevented. Despite all these advancements, a healthy and peaceful pregnancy journey can thus prevent various health issues for both the mother and the baby. So enjoy pregnancy and motherhood.
