What is nuchal translucency screening?
Screening of the nuchal translucency is an ultrasound marker of prenatal diagnosis, which measures the 'translucent' space of the fetal cepha region (anatomical region between the tegument and the cervical spine).
This marker is useful in formulating the diagnosis of Down syndrome or other congenital abnormalities such as congenital heart disease (heart malformations).
Fetuses with congenital anomalies tend to accumulate more fluid in the posterior cervical region (neck) during the first trimester of pregnancy.
Ultrasound, the anatomical region appears thickened, enlarged, but can not confirm a true diagnosis, being only a marker of suspicion.
Further investigations such as amniocentesis or biopsy of chorionic villi are necessary to formulate a diagnosis of certainty.
Ultrasound prenatal screening that can evaluate the nuchal translucency is increasingly used in our country, being available in most university centers where there are specialized medical equipment and staff.
Not every specialist in medical imaging (ultrasound), can perform such prenatal ultrasound screening, with some accreditation in this area. Also, a high performance device is needed, which can easily identify and quantify certain congenital anomalies at small gestational ages (12-16 weeks).
Any pregnant woman who wants to take the test should be interested in the best place to do this, especially since the investigation is not painful and does not present any danger to the baby or the mother.
How is the translucency screening performed?
The screening of the nuchal translucency is carried out at a gestational age between weeks 11-14 of pregnancy (optimal in week 12), but never after week 14, because the test becomes irrelevant after this age.
The specialist performing the ultrasound for the screening of the nuchal translucency must first determine the exact gestational age. This is not very difficult and requires measuring the shaft that connects the crest with the nothingness (the seat). After determining the age of the fetus, the thickness of the nuchal region is measured, which is modified in certain chromosomal abnormalities.
The obtained values are then correlated with certain statistical indices, formulating the real risk of the existence of a genetic anomaly such as Down syndrome. Taking into account the fact that the nuchal region thickens from day to day, several studies were carried out that established its thickness for each day of the gestational interval between week 11 and 14 of pregnancy.
The information obtained after the translucent ultrasound can be correlated with other ultrasound markers and also with some personal factors (mother's age, chromosomal diseases in the family, etc.). The older the pregnancy is, the greater the risk of developing a birth defect.
If the ultrasound values are obtained immediately, the final result can only be available after a certain period of time, directly proportional to the doctor's experience, the statistical data that he possesses as well as his personal medical experience.
It is often indicated that the final result should be interpreted by the specialist genetic counselor, as he has extensive experience and can determine if the subsequent diagnostic algorithm is required.
Interpretation of nuchal translucency screening
The result of the ultrasound screening of the nuchal translucency is actually a statistical expression, which correlates several factors, namely:
For example, a 35-year-old pregnant woman has a 1 in 270 risk of giving birth to a child with Down syndrome. This risk increases proportionally with the age of the woman. Thus, regardless of the situation, one in 270 women 35 years old will have a child with this serious genetic condition.
The prognosis for age is quite worrying, so every woman over the age of 35 who wants a child should consider this.
The risk of a 35-year-old pregnant woman having a Down syndrome child remains 1 to 270, even if the values of the nuchal translucency are normal! If the test is abnormal, however, the risk of having a child with a chromosomal abnormality increases significantly.
Screening of the nuchal translucency cannot make a diagnosis of certainty, often false-positive or false-negative results can often be obtained, this means that not every pregnant woman who has the modified test will have a child with a chromosomal abnormality and not every pregnant woman, respectively. with a normal test she has the guarantee that a normal child will be born.
The pregnant woman should seek the advice of a genetic specialist who can present the exact risks of a pathological pregnancy (with chromosomal abnormalities). Based on previous medical studies and statistics in the field, he will decide with the pregnant woman if it is recommended or necessary for her to carry out other screening tests or prenatal diagnosis.
The specialist in genetic counseling will explain to the pregnant woman the risks of investigations that can confirm a suspected diagnosis (amniocentesis or biopsy of chorionic villi), but also the implications of the birth of a child with an incurable chromosomal abnormality.
The values of the nuchal translucency are also modified in other congenital anomalies besides Down syndrome, such as congenital heart abnormalities. In this case it is recommended to perform an ultrasound that evaluates the anatomy and function of the fetal heart (echocardiography).
If a congenital heart abnormality is identified, the fetus will require continuous monitoring throughout the pregnancy and also specialized medical care immediately after birth. Some congenital heart malformations can be corrected surgically, immediately after birth or after a few weeks or months after birth.
Take as an example a 40-year-old woman who wants a child. She has an initial risk of 1 in 77 giving birth to a child with Down syndrome. At a gestational age of 11 weeks and 2 days, he performs the screening of the nuchal translucency, the doctor who performs the ultrasound, revealing a narrow, thin (normal) nucleus area.
Coreland mother's age, fetal age and ultrasound values of nuchal translucency, the doctor will achieve a final result equivalent to a risk of 1 to 761. The risk of her fetus having a trisomy 18 is even lower, from 1 to 10,000, for this reason investigations later such as amniocentesis or biopsy of chorionic villi is not mandatory, the risk of a congenital anomaly being lower than that of a spontaneous abortion caused by an invasive investigation.
80% accuracy in identifying Down syndrome
What do we mean by: "Does the test have 80% accuracy in identifying Down syndrome?"
An 80% accuracy in identifying Down syndrome means that, in the case of a pregnant woman who has a fetus with this condition, there is an 80% possibility that the test will identify the disease and 20% will not do it (false negative). So, there is a 20% chance that Down syndrome will not be diagnosed by this method and thus the woman will give birth to a child with this condition.
Also, the inclusion in the 80% does not necessarily mean the presence of Down syndrome (false-positive result), which will be confirmed later by amniocentesis. The major disadvantage of the method is that it can give many false-positive and false-negative results.
The rate of false-positive results is about 5%. The false-positive result means that the screening test is modified even if in fact there is no congenital anomaly.
However, this can only be confirmed by amniocentesis, the only method that can make a diagnosis of certainty.
In order to increase the specificity of the nuchal translucency as the screening method at the end of the first trimester, it is recommended the concomitant use of some blood tests performed during the gestational interval 11-14 weeks. The accuracy of Down syndrome detection increases from 80% to 90%, but it cannot be considered a diagnosis, the diagnosis of certainty being also established by amniocentesis.
The combined test is increasingly used in us in the country and is available in most university hospitals. Blood tests measure the serum level of 2 freebeta-hCG and PAPP-A proteins. A few drops of blood are sufficient to be analyzed in the laboratory, the result being available in 24 hours.
Some centers instead use quadruple screening or a combination of these, nuchal translucency and free beta-hCG and PAPP-A determination. The concomitant use of the three screening methods increases the accuracy of Down Syndrome screening to 95%.
A biochemical examination that shows a decrease in the plasma protein concentration associated with pregnancy (PAPP-A), the increase of the serum level of the free subunit of HCG (freebeta-hCG) with an ultrasound mark of nuchal translucency greater than 3 millimeters and a modified quadruple test. , can establish a positive diagnosis of Down syndrome in 95% of cases.
In recent years, a series of new ultrasound markers have been added to prenatal screening for Down syndrome, which increase the accuracy of the diagnosis of this condition. Of these we mention the ultrasound of the nasal pyramid (of the nasal bone), investigation that is recommended between weeks 11-14 of gestation, increases the accuracy of the diagnosis of Down syndrome up to 97% (if used concurrently with the other screening methods described above).
Advantages of nuchal translucency screening in the first trimester of pregnancy
The major advantage of determining the translucency of the nose is that this screening method is a non-invasive one, quite cheap and easy to perform, and can provide important information regarding the possibility of the presence of Down syndrome in the fetus.
The method is often used in conjunction with other prenatal screening investigations and identifies pregnant women at increased risk of having a child with a congenital anomaly. Only pregnant women with modified tests will be exposed to the risk of having a miscarriage after performing another invasive investigation such as biopsy of chorionic villi or amniocentesis.
Pregnant women with pathological changes of the testis associated with a nuchal translucency of more than 3 millimeters, may opt for a biopsy of chorionic villi that can be performed as early as week 11-12 of pregnancy and which can confirm the suspected genetic diagnosis.
Nuchal translucency screening is not an invasive investigation and is performed with ultrasound. This is among the first fetal ultrasound markers that can be analyzed in the context of a chromosomal genetic disorder.
Starting with the 18-20 week of gestation, other ultrasound markers can be highlighted which may indicate the presence of a congenital anomaly in the fetus (measurement of the femur, identification of congenital anomalies of the heart, malformations of the digestive tract and intestines, presence of cysts in the brain, identification of calcifications in the musculature of the heart or the presence of malformations of the fetal kidney).
Disadvantages of nuchal translucency screening
As with any prenatal screening test, nuchal translucency screening is for guidance and not diagnostic. It cannot accurately determine the diagnosis of a congenital anomaly.
False-positive results can often cause anxiety and increased anxiety in pregnancy, which is forced to do other investigations that can confirm or disprove the suspected diagnosis.
Also, we must not forget that screening for nuchal translucency cannot detect neural tube defects (spina bifida or anencephaly) as other screening tests do, such as the triple test.
The identification of the ultrasound markers that can signal the presence of a congenital anomaly is superior in the second trimester of pregnancy.