The thickness of the collar space - why is it measured and what does it mean?

The nuchal translucency (or cervical / nuchal translucency) is an area with an accumulation of viscous fluid in the back of the fetal neck between the skin and the surface of the soft tissues surrounding the cervical spine. With chromosomal abnormalities, its volume increases, which is manifested by an increased thickness of the collar space.

nuchal translucency thickness (NUT) is measured at the first screening, which is performed at the end of the first trimester of pregnancy. The procedure is a non-invasive research method: it is carried out using ultrasound - measured by reflected ultrasound waves from the liquid and solid media of fetal tissue.

Determination of TVP using ultrasound

During pregnancy, women undergo an ultrasound examination and use it to determine TVP. The procedure is prescribed once per trimester and is aimed at identifying pathologies. To determine them, echographic markers are measured in the fetus. The obtained parameters correspond to the norm. The most important study is in the first trimester, after which the marker data loses relevance.

During the first trimester the following is measured:

• limb length;
• profile bones;
• thickness of the collar space.

To carry out the last of these measurements, the ultrasound specialist uses a sensor to direct a sound wave to the neck area under the back of the head or to the first spine and takes readings. In this place, between the skin and bone tissues of the embryo, subcutaneous fluid accumulates. Its presence may indicate both a normally developing pregnancy and abnormalities in the unborn child. Later, the amount of liquid gradually decreases, and the result of the study loses its information content.

For accurate diagnosis, when measuring the thickness of the collar space, several conditions must be met:

• The pregnant woman is in a lying position;
• The fetus is placed in a convenient position for examination. A lateral position is considered suitable when the profile can be analyzed. If the desired position is not occupied, then a transvaginal examination is performed.
• Coccygeal-parietal size ratio;
• Compliance with diagnostic deadlines;

In addition, the specialist must be competent in the matter of TVP. The apparatus used to carry out the diagnosis also plays an important role.

When measuring a collar, two rules are also observed:

• Taking into account the thickness of the amniotic membrane;
• Using Doppler.

Norms for the thickness of the collar space

Medical statistics on the thickness of the nuchal translucency at different stages of pregnancy are presented in the following table.

Important Values ​​exceeding the 95th percentile are considered deviations from the norm.

How accurate is the result?

Between 65 and 85% of trisomy fetuses will have increased nuchal translucency thickness.1

For example, the TVP scan itself detects Down syndrome in approximately 62% of cases. Although in 5% of those examined, this diagnosis is subsequently removed (false positive result). (2)

To more accurately determine the risk of Down syndrome, the results of ultrasound of the nuchal area should be combined with the results of maternal blood tests for the presence of free hCG and PAPP-A protein. This combination already shows the detection of this syndrome in 73% of cases and 4.7% of false-positive results, including. Muller F. et al. “First trimester screening for Down syndrome in France, combining measurement of fetal cervical translucency and biochemical markers.” Prenatal diagnosis. Oct 2003 23(10):833-6. [go][/ref] In another study, these figures were 79.6% and 2.7%, and with the addition of ultrasound in the second trimester - to 89.7% and 4.2%, respectively. Another study showed a detection rate of 88% for trisomy 21 (Down syndrome) and 75% for trisomy 18 (Edwards syndrome), with a false positive rate of 3.3%.

When ultrasound and blood test results are positive, chorionic villus sampling or amniocentesis is required to confirm the presence of a genetic abnormality.

In the past, a blood test was performed first, and if abnormal, a scan was ordered if possible. Now parallel examination is practiced and all tests are prescribed at the same time, which has made it possible to reduce the number of false positive results.1

The likelihood of developing chromosomal diseases

The likelihood of chromosomal abnormalities is directly proportional to the increase in TVP, and the most common pathologies are:

1. Down syndrome
   (trisomy 21, more than 50%);
2. Edwards syndrome
   (trisomy 18, ?25%);
3. Turner syndrome
   (monosomy on the X chromosome, ?10%);
4. Patau syndrome
   (trisomy 13 chromosome, ?5%), etc.

Trisomy

is the presence of an additional chromosome, and
monosomy
is the absence of a chromosome in the chromosome set.
The dependence of chromosomal abnormalities on the value of TVP (research by P. Pandya
) is presented in the table.

Consequences of expansion of the nuchal space in the fetus

Unfortunately, in most cases the diagnosis is confirmed and the child is born with some kind of disease . There are many unpleasant consequences of increased VP, for which future parents should be prepared.

• Heart defect in the fetus.
• Developmental delay.
• Anomaly in the development of the gastrointestinal tract (gastrointestinal tract) and genitourinary system.
• Deviations in the development of the skeleton and muscles.
• Impaired swallowing reflex, breathing problems.
• Down syndrome . In this case, the child has one more chromosome than ordinary people (47 instead of 46, trisomy 21 chromosomes instead of a pair.). This leads to slow development, abnormal structure of the face, skeleton, disruption of the functioning of some organs and the psyche.
• Patau syndrome . The presence of 47 chromosomes in a child (trisomy 13 chromosomes), extra fingers, disruption of the brain and internal organs, abnormal facial structure.
• Turner syndrome . Here there is an absence of the X chromosome, which results in the child’s short stature, short life expectancy, skeletal deformation, skin pigmentation, mental retardation, etc. Found only in females.
• Edwards syndrome . The child has an extra chromosome (trisomy 18), short stature, problems with the heart and mental development.
• Triploidy. Children with this diagnosis have three copies of each chromosome, instead of two. In most cases, the baby does not survive to birth.
• Death of the fetus.

If TVP is not normal

At the moment, the optimal way to form a risk group for chromosomal abnormalities is a combined examination, including determination of TVP by ultrasound, analysis of biochemical markers ( β-hCG
PAPP- A in the blood serum of a pregnant woman) and age-related screening
.
After this, a conclusion is issued, which notes the risk for chromosomal abnormalities (for example, the combined risk for Down syndrome is 1:300). If there is a high risk, a woman is recommended to undergo amniocentesis
(sampling of amniotic fluid) to determine the chromosome composition of the fetus.
Cordocentesis
(cord blood sampling) or
chorionic villus biopsy
(chorionic villus sampling) may also be offered

Information It should be remembered that these examination methods are carried out only with the informed consent of the pregnant woman, and every woman has the right to refuse them!

With a normal chromosome set (23 pairs of chromosomes), pregnancy is prolonged as usual. If there are severe chromosomal diseases that are incompatible with life, the woman is strongly recommended to terminate the pregnancy. If Down syndrome is present, the family can independently decide on the future fate of the unborn child (prolong the pregnancy or terminate it). Thus, the determination of TVP is fundamental during ultrasound of the first trimester, as it makes it possible to timely diagnose the chromosomal pathology of the fetus and make the right and informed decision about each specific pregnancy.

If the thickness of the collar space is increased

With an increased TVP, a woman is included in the risk group. Depending on other indicators, this will be a medium or high group. The most common abnormalities associated with TVP expansion are chromosomal:

• Down syndrome
• Patau syndrome
• Turner syndrome
• Edwards syndrome.

In addition to the incorrect number of chromosomes, the size of the neck fold can be affected by:

• Heart defects
• Improper lymph circulation
• Anomalies in the development of connective tissues
• Deviations in the formation of bone structures

This may also be due to a complicated pregnancy:

• Anemia
• Taxoplasmosis
• Cytomegalovirus
• Parvovirus.

If the result is unfavorable, the woman is recommended to undergo an ultrasound scan every four weeks to monitor growth or, conversely, a decrease in fluid in the cervical spine. In addition, future mothers who are at risk are given a genetic consultation to clarify possible pathologies.

The specialist examines the medical history and calculates the probability percentage by correlating:

• ultrasound indicators;
• the result of a biochemical blood test;
• mother's age.

If TVP is increased, an additional blood test and repeat ultrasound examination may be prescribed. With secondary confirmation and a risk ratio of 1:300, further studies are recommended:

• Amniocentosis. Collection of amniotic fluid;
• Chorionic biopsy. Taking the cells that form the placenta for analysis;
• Placentobiopsy. Collection of placental cells;
• Cordocentesis. Taking umbilical cord blood or material from embryonic villi for analysis.

The result of each procedure, unlike the previous ones, gives one hundred percent clarity on the presence of TVP pathologies in the fetus.

After receiving the data, the pregnancy is prolonged or recommendations are given for termination for medical reasons. The choice remains with the future parents.

Exceeding the norm and additional examination

Carrying out an ultrasound examination of a pregnant woman to measure the thickness of the collar space is mandatory in the following cases:

• Sagittal scan of the fetal head
• 10 to 13 weeks of pregnancy

If, based on the results of ultrasound diagnostics, it is determined that the acceptable parameters are exceeded, the woman is prescribed a more thorough comprehensive examination.

It includes:

• Clinical examination
• Repeated examination of the thickness of the collar space with the measurement of additional indicators
• by biochemical markers

If, based on the results of a complete additional examination, a high probability of chromosomal abnormalities of the fetus is established, the pregnant woman is included in the risk group. To determine the genetic makeup of the embryo, invasive prenatal diagnostics is performed by

transabdominal sampling of material:

• Chorionic villus biopsy - collection of cells that form the placenta
• Placentobiopsy – collection and examination of cells
• Amniocentesis - through puncture of the amniotic sac, amniotic fluid is collected
• Cordocentesis - collection of blood fluid from the umbilical cord or collection of biomaterial from the villi of the embryonic fetus

Based on the results of all combined examinations, the chromosome set is determined. If their number is 23 pairs, the fetus is preserved and the pregnancy continues.

If there are critical indicators, a decision is made to terminate the pregnancy.

The most common anomalies depending on TVP at 12 weeks of pregnancy:

• Turner syndrome
• Edwards syndrome
• Patau syndrome

The decision to terminate pregnancy or prescribe drug therapy is made exclusively by the doctor. Only he can calculate the probability of having a baby with genetic disorders. But the final decision to prolong the pregnancy or not remains solely with the woman.

A diagnostic examination of the thickness of the nuchal translucency is a significant manipulation in monitoring the development of the embryo. Determining the pathology coefficient allows for timely detection of abnormal fetal development.

You will learn about diagnosing Down syndrome from the video:

• Leakage of amniotic fluid in the first and second half...
• Amniotic fluid leaks during pregnancy: reasons,…

Why is the first ultrasound performed at 12 weeks of pregnancy? And all because it is during this period that signs of serious chromosomal abnormalities (Down syndrome) are most likely to be identified. Diagnostic criteria: thickness of the collar space, length of the nasal bone in the embryo and other measurements. What are the acceptable limits for these indicators, and what to do if the result differs from the norm? Find answers to these and many other questions below.

Features of the thickness of the collar space at 12 weeks

At the twelfth week, the embryo has already passed three months of development and the main markers by which conclusions can be drawn about its development are visible to the specialist. This is the most informative stage for identifying various pathologies and abnormalities in the development of the fetus, as well as during pregnancy in general. Each parameter has its own data, which is correlated with the norm table. As for the collar zone, its thickness also has certain limits. The accumulation of fluid that gives rise to a fold in the twelfth week should not exceed 3 millimeters, the minimum size is 0.7 mm. It is important to keep in mind that the data may vary depending on the location of the fetal head: whether it is pressed to the chest, turned to the side, etc.

TVP values ​​at 12 weeks of pregnancy

So, based on the data given in our table, we can conclude that if a woman is 12 weeks pregnant, then the TVP size will be 1.6 mm (normal on average). The minimum norm for TVP at this period is 0.7 mm, and the maximum limit of the normal value is 2.5 mm. What does an increase in TVP levels above normal mean at 12 weeks of pregnancy? In this case, the risk that genetic (chromosomal) abnormalities and congenital organ defects will be detected in the embryo greatly increases.

Comparison of normal and abnormal nuchal translucency thickness

In 95% of cases, a fetus with Down syndrome exhibits a TVP value greater than 3.4 mm. A TVP size of more than 5.5 mm at 12 weeks in 70% of cases means Edwards syndrome.

Features of TVP at 13 and 14 weeks

In the second trimester, at 13-14 weeks of pregnancy, the cervical fold decreases in size. The indicator is gradually becoming irrelevant. At week 13, in the form with the baby’s data, in the line with data on TVP, an indicator from 1.7 to 4 mm can be entered. These figures are within the accepted framework and the risk of developing pathology in the embryo or pregnancy complications in the mother is reduced to a minimal percentage.

Expansion of the collar zone does not always indicate the development of problems in the fetus. It is important to know that its dimensions always correlate with those of other echographic markers obtained during ultrasound examination. If, however, the size of the TVP goes beyond the normal limits, you should not despair. There can be many reasons. In each specific case, they are determined by a specialist with specific qualifications. And depending on the detected anomalies, further actions may be prescribed. It is worth remembering that the result of measuring the collar area is relevant only during the first trimester. Try not to miss this time.

What is TVP?:

The nuchal translucency thickness (NST) is the space between the inner edge of the fetal skin and the soft tissues of the neck. On an ultrasound at 11-13 weeks, this place is visible as a black (anechoic) stripe. All embryos have a collar space; its size ranges from 0.7 to 2.7 mm, depending on the stage of pregnancy. A thickening of more than 3 mm does not indicate pathology, but only serves as an indication for a more detailed examination. Such women are included in the risk group for giving birth to babies with hereditary pathology.

It is important to do the first ultrasound within the specified period, since before the 11th week the fetus is very small, and it is difficult to examine the necessary indicators. And after 14 weeks, the collar space changes its size due to the accumulation of fluid. This significantly reduces the diagnostic value of screening for hereditary diseases and developmental defects. The measurement accuracy depends on:

The doctor’s qualifications are the ability to “catch” the fetus in the sagittal plane with a neutral position of the head of the coccygeal-parietal size (CPR) of the fetus from 45 to 84 cm.

Fluctuations in TVP values ​​of half a millimeter may indicate a change in the position of the fetus. For example: when the head is flexed, the size is 0.4 mm smaller, and when the head is extended, it is 0.6 mm larger. Both sizes are normal.