The first trimester screening test, also known as the dual screening test or the 11-14 test, is a screening test to detect babies with Down syndrome and a chromosomal abnormality called Trisomy 18 at a very early stage of pregnancy. No matter how old they are, all women are at risk of giving birth to a physically or intellectually disabled baby. The risk of giving birth to a baby with Down syndrome is 1 in 1530 for a 20-year-old woman, while this risk increases to 1 in 30 for a 44-year-old woman.
As with all screening tests, this test does not make a diagnosis. It only indicates babies at high risk of the disease and ensures that diagnostic tests are performed in these babies that lead to a definitive diagnosis. In other words, just as a high risk test does not prove that the baby has an anomaly, a low risk does not guarantee that the baby is completely healthy.
The first trimester screening test has some advantages compared to the triple test. The most important of these is that it allows for an earlier and risk-free termination of pregnancy in the event of a possible negative as a result of the test being performed at an earlier period. Moreover, its sensitivity is higher than the triple test and helps to recognize 90% of Down syndrome and trisomy 18 cases.
The Dual Test ( 1.Trimester ) How is the Down Syndrome Test applied?
the 11-14 test is basically done by evaluating two separate examinations together. Dec.11-14 test is basically done by evaluating two separate examinations together. These:
Measurement of the thickness of the liquid part located behind the baby’s nape by ultrasound (fetal nape thickness)
It is the measurement of the free portion of the pregnancy hormone β-hCG (free β-hCG) and another protein called PAPP-A (pregnancy-specific plasma protein-A, pregnancy associated plasma protein-A) in a blood sample taken from the mother.
When these measurements are performed alone, their sensitivity is low, but when they are evaluated Decisively, the chance of success is up to 90%.
Fetal nuchal thickness
Fetal nuchal thickness is a term used to describe the part of the back of the baby’s neck that appears in dark color on ultrasound. The original form of the term in English is nuchal translucency. As the pregnancy progresses and the baby grows, the thickness of the nape gradually increases. Therefore, the measurement is 11-14. it can be done between weeks and requires great Decisiveness. A millimeter error during the measurement can cause a large change in the risk ratios.
A lot of research has been done between 11 and 14. it has been revealed that there is a close relationship between the thickness of the fetal Decap Decapitation between the weeks of pregnancy and some chromosomal abnormalities, especially Down syndrome. In various studies, it has been found that 40-70% of babies with Down syndrome can be detected only by measuring the thickness of the fetal nape during the specified time period. However, it should be borne in mind that the mothers of these babies are already high-risk pregnant women who have been examined due to their advanced age pregnancies or a history of giving birth to a baby with chromosomal abnormalities in previous pregnancies.
Studies conducted in women in the low-risk group, on the other hand, have yielded conflicting results. The underlying reason for this contradiction is the differences that arise between the people Deciphering the measurement, even if the same person Deciphers the measurement, between the two measurements. In addition, consensus on the definition of increased fetal thickness has not been achieved for a long time. October. Which section of the ultrasound should be used when measuring the thickness of the fetal nape has also been the subject of discussion for a long time, it has been suggested that the sensitivity of different sections is higher.
According to the widely accepted opinion today, pregnancy occurs on the 11th-14th. if the nuchal thickness is more than 3 millimeters in the section where the baby’s head-butt length is measured between weeks, it is considered to be an increased fetal nuchal Decollete thickness.
Fetal nuchal thickness does not increase only in chromosomal abnormalities. In studies, it has been shown that increased fetal nuchal thickness increases mainly in cardiac anomalies of the baby, along with some other genetic disorders. Heart anomalies of the baby are detected by detailed ultrasonography performed in the second trimester. 50-90% of babies with chromosomal disorders also have abnormalities in the heart and large vessels. For this reason, it has been suggested that the main reason for the increase in nuchal thickness that occurs in chromosomal disorders is actually a concomitant heart anomaly.
The cases in which the fetal nuchal thickness may be more than normal are as follows
Chromosomal disorders
Heart anomalies
Lung anomalies (diaphragmatic hernia)
Kidney anomalies
Abdominal wall anomalies (omphalocele, gastroschisis)
Some genetic diseases (Arthrogryposis, Noonan syndrome, Smith-Lemli-Opitz syndrome, Stickler syndrome, Jarcho-Levin syndrome and some skeletal anomalies
There are some drawbacks to the use of fetal nuchal thickness measurement alone in the detection of chromosomal disorders at an early stage. Considering that many abnormal pregnancies result in miscarriage, chorionic villus sampling, which will be performed after a false positive test, will increase the risk of miscarriage in a normal baby. On the other hand, in the presence of mosaicism, where some of the cells are normal and some are abnormal, the appearance of only abnormal cells in villus sampling will lead to the termination of the life of a baby who can continue his life normally. In addition, chor October villus sampling performed at an early stage is both more difficult and more expensive than amniocentesis performed at a later stage. Much more important than these is the experience of the person who does the revenge. Since the measured values are at the level of one tenth of a millimeter, the slightest error will cause significant changes in the risk values. For all these reasons, the cost-effectiveness ratio of fetal nape measurement performed alone is not satisfactory.
Positive result of the test
A positive double test does not necessarily mean that the baby has a chromosome disorder. A positive test only indicates that the risk is high in that baby and further diagnostic tests should be performed. Further examinations are detailed ultrasonography, chorionic villus sampling and amniocentesis. You need to decide together with your doctor which test will be suitable for you.
Negative result of the test
The fact that the risk is low in the test, that is, negative, does not guarantee that the baby does not have a chromosome disorder. It only shows that the risk to the baby is not greater when compared with women of the same age group in the general population. In addition, the dual test only determines the risk in terms of chromosome disorders. It does not identify a risk in terms of neural tube defects. 16-20 to determine this risk. a triple test can be done in weeks. However, since a significant part of neural tube defects can be detected by ultrasound, there are also opinions suggesting that only a detailed ultrasound will be sufficient instead of a triple test in the second trimester in people who have undergone a double test. A consensus has not yet been formed in scientific circles on this issue.
American Association of Obstetricians and Gynecologists (ACOG) at the time of birth, mother’s age 35 or a more advanced case, instead of screening tests, diagnostic tests such as amniocentesis or chorionic villus sampling in conjunction with genetic counseling koriyo recommends. The reason for this is that screening tests only determine the risk, do not definitively reveal the presence or absence of the condition. On the other hand, a double test or triple test determines the risk of only one group of chromosomal abnormalities, and does not give an idea of other anomalies that are more common than usual in this age group.