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 Down syndrome and a chromosomal abnormality called 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. The risk of giving birth to a baby with Down syndrome increases from 1 in 1530 for a 20-year-old woman 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 infants at high risk for the disease and provides diagnostic tests that lead to a definitive diagnosis in these infants.
In other words, the high risk of the test is not evidence of an anomaly in the baby, 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 to terminate the pregnancy 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.
How is the test administered?
The 11-14 test is basically performed by evaluating two separate examinations together. These:
- Measuring the thickness of the liquid part behind the baby’s neck with ultrasound (fetal nuchal translucency)
- It is the measurement of the free part 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 the blood sample taken from the mother.
When these measurements are made alone, their sensitivity is low, but when they are evaluated together, the chance of success is up to 90%.
Fetal nuchal thickness
Fetal nuchal translucency 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. can be done between weeks and requires great attention. A millimetric error to be made while making the measurement can cause a big change in the risk ratios.
Numerous studies have shown that there is a close relationship between fetal nuchal translucency between 11 and 14 weeks of gestation and some chromosomal anomalies, especially Down syndrome. It has been shown in different studies 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 women in the low-risk group 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, consensus on the definition of increased fetal thickness could not be reached 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, 11-14 days of pregnancy. An increased nuchal nuchal nuchal thickness is considered as a nuchal nuchal translucency that is more than 3 millimeters in the section where the head-rump length of the baby is measured between weeks of gestation and gestational age.
Fetal nuchal nuchal thickness does not increase only in chromosomal anomalies. 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 performed 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:
- 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
The use of fetal nuchal translucency measurement alone in the early detection of chromosomal disorders has some drawbacks. 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 right for you.
Negative result of the test
Having a low risk, ie negative, in the test does not guarantee that the baby does not have a chromosome 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 part of neural tube defects can be detected by ultrasonography, there are also opinions suggesting that detailed ultrasound alone would be sufficient instead of triple testing in the second trimester in people who had double tests. There is no consensus in scientific circles on this issue yet.
The American Association of Obstetricians and Gynecologists (ACOG) recommends diagnostic tests such as amniocentesis or chorionic villus sampling together 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, the double test or triple test only determines the risk for a group of chromosomal anomalies and does not give an idea about other anomalies that are more common in this age group.