The first trimester screening test, also known as the double screening test or the 11-14 test, is a screening test to detect babies with a chromosomal abnormality called Down’ and Trisomy 18 in the very early stages of pregnancy. Regardless of age, all women are at risk of giving birth to a physically or mentally handicapped baby. While the risk of giving birth to a baby with Down syndrome is 1 in 1530 for a 20-year-old woman, this risk increases to 1 in 30 for a 44-year-old woman.
As with all screening tests, this test is not diagnostic. It only indicates infants at high risk for the disease and provides diagnostic tests that lead to a definitive diagnosis in these infants. In other words, a high risk test does not prove that the baby has an anomaly, and 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 the test is performed in an earlier period, and in case of a possible negativity, it allows the pregnancy to be terminated earlier and without risk. Moreover, its sensitivity is higher than the triple test and helps to identify 90% of Down syndrome and trisomy 18 cases.
Double Test (1st Trimester) Down Syndrome How is the test applied?
11-14 test is basically done by evaluating two separate reviews together. These are:
Measuring the thickness of the liquid part behind the baby’s neck by ultrasound (fetal nuchal thickness)
Name of the free part of the pregnancy hormone β-hCG (free β-hCG) and PAPP-A (pregnancy-specific plasma protein-A, pregnancy associated plasma protein-A) in the blood sample taken from the mother measurement of another given protein.
Fetal nuchal thickness
Fetal nuchal nuchal thickness is a term used to describe the dark colored part of the back of the baby’s neck in ultrasonography. The original form of the term in English is nuchal translucency. As the pregnancy progresses and the baby grows, the nuchal thickness gradually increases. Therefore the measurement is 11-14. It can be done between weeks and requires great attention. A millimetric error in the measurement may cause a large change in the risk ratios.
Many studies have shown that there is a close relationship between fetal nuchal translucency between the 11th and 14th weeks of gestation and some chromosomal anomalies, especially Down syndrome. In different studies, it has been revealed that 40-70% of babies with Down syndrome can be detected by measuring fetal nuchal translucency only in the specified time period. However, it should be kept in mind that the mothers of these babies are already high risk pregnancies, who are included in the examination due to advanced age pregnancies or a history of giving birth to a baby with chromosomal anomalies in previous pregnancies.
Studies in low-risk women have yielded conflicting results. The underlying reason for this discrepancy is the differences between the two measurements, even when the same person is measuring. In addition, there was no consensus on the definition of increased fetal thickness for a long time. Which section of the ultrasound should be used when measuring fetal nuchal translucency has been a matter of debate for a long time, and it has been suggested that the sensitivity of different sections is higher.
According to the widely accepted view today, the 11-14th day of pregnancy. An increased nuchal nuchal nuchal thickness is considered to be more than 3 millimeters in the section where the baby’s head-butt length is measured between weeks of gestation.
Fetal nuchal nuchal thickness does not increase only in chromosomal abnormalities. In studies, it has been shown that increased fetal nuchal thickness increases mainly in fetal heart anomalies along with some other genetic disorders. Heart anomalies of the baby are detected by detailed ultrasonography in the second trimester. In 50-90% of babies with chromosomal disorders, anomaly also occurs in the heart and great vessels. For this reason, it has been suggested that the main reason for the increase in nuchal thickness in chromosomal disorders is actually a concomitant cardiac anomaly.
Conditions where fetal nuchal translucency may be greater than normal are as follows
Some genetic diseases (Arthrogryposis, Noonan syndrome, Smith-Lemli-Opitz syndrome, Stickler syndrome, Jarcho-Levin syndrome and some skeletal anomalies
There are some drawbacks to using the fetal nuchal nuchal thickness measurement alone in the early detection of chromosomal disorders. Considering that many pregnancies with anomalies result in miscarriage, chorionic villus sampling to be performed after false positive test will increase the risk of miscarriage in a normal baby. On the other hand, in the presence of mosaicism in which some of the cells are normal and some are abnormal, the detection of only abnormal cells in the villus sampling will cause the death of a baby who can lead a normal life. In addition, chorionic villus sampling performed in the early period is both more difficult and more expensive than amniocentesis performed in later periods. Much more important is the experience of the person making the measurement. Since the measured values are at the level of one tenth of a millimeter, the slightest mistake will cause significant changes in the risk values. For all these reasons, the cost-effectiveness ratio of fetal nuchal translucency measurement alone is not satisfactory.
Positive result of the test
A positive double test does not necessarily mean that the baby has a chromosomal disorder. A positive test only indicates that the risk is high in that baby and further diagnostic tests are required. Further investigations mean detailed ultrasonography, chorionic villus sampling and amniocentesis. You and your doctor need to decide which test is appropriate for you.
Negative result of the test
A low risk, ie negative, in the test does not guarantee that the baby does not have a chromosomal disorder. It simply indicates that the risk to the infant is not greater when compared to women of the same age group in the general population. In addition, the double test only determines the risk for chromosomal disorders. It does not determine a risk for neural tube defects. 16-20 to determine this risk. Triple testing can be done in weeks. However, since a significant portion of neural tube defects can be detected by ultrasonography, there are also opinions suggesting that detailed ultrasound alone will be sufficient instead of triple testing in the second trimester in people who have double-tested. There is no consensus on this subject in scientific circles yet.
The American Association of Obstetricians and Gynecologists (ACOG) recommends performing diagnostic tests such as amniocentesis or chorionic villus sampling with genetic counseling instead of screening tests if the maternal age is 35 or more at the time of birth. The reason for this is that screening tests only determine risk and do not definitively reveal the presence or absence of the condition. On the other hand, double test or triple test determine the risk for only one group of chromosomal anomalies and do not give an idea about other anomalies seen more than normal in this age group.