Types of Prenatal Genetic Testing: What Pregnant Women Need to Know
11.07.2023 20:57
Prenatal genetic testing is a test performed during pregnancy to detect fetal developmental defects. The latest technologies in genetics allow us to find out in advance whether the child will be healthy by looking at the embryo's DNA.
There are invasive and non-invasive methods of genetic testing. In the first case, there is a medical intervention in the body. It carries a very small, but still real risk of miscarriage. Such methods include chorionic villus sampling and amniocentesis. They have an alternative - non-invasive methods that are completely safe. Among them, NIPT - non-invasive prenatal testing and diagnostic testing are popular. Let's consider the pros and cons of each method and tell you how to choose the one that suits you.
Non-invasive prenatal testing (NIPT)
How NIPT is performed
A small amount of blood from a vein is used for the analysis. The scientific basis for the method is simple. The fetus's DNA molecules enter the mother's bloodstream from the cells that form the placenta (trophoblast). In the laboratory, the child's DNA can be isolated from the pregnant woman's blood. The analysis does not require any special preparation.
The concentration of the fetus's genetic material increases as it develops. Starting from the 9th-10th week, it becomes sufficient for study. That is, NIPT gives a chance to learn about chromosomal pathologies of the fetus even before the first screening, which is carried out at the 12th week.
This type of testing has become increasingly popular in the last decade and does not pose any harm to the woman or the fetus.
Photo: Pixabay
Obstetrician-gynecologist EVACLINIC Alexandra Rakovich :
In fact, the risk of miscarriage or frozen pregnancy after amniocentesis increases by only 0.5% of the general population. It is important that the expectant mother has no contraindications and that she is properly and well prepared for the examination under the supervision of pregnancy management doctors. It is important to consider that NIPT is not included in the protocols, so it can only be performed on a paid basis. But if a geneticist recommends an invasive test, NIPT cannot always serve as a replacement for it.
What genetic abnormalities can be detected with NIPT?
The basic test is used to determine Down syndrome. This disease is associated with trisomy 21 chromosome: instead of a normal pair of chromosomes, there are three of them. The accuracy of this test is 99 percent. Basic NIPT is recommended for older mothers (from 35 years old): they have a statistically higher risk of conceiving a child with Down syndrome.
Enhanced NIPT detects other common genetic abnormalities:
Edwards syndrome;
Patau syndrome, Turner syndrome;
Klinefelter syndrome;
Jacobs syndrome;
trisomy X.
The most extensive NIPT is aimed at identifying microdeletions. These are small abnormalities in the structure of chromosomes - the absence or damage of their fragments. Rare diseases are associated with such genetic defects. These are Angelman, Prader-Willi, and cat cry syndromes. Although these are not common diseases, they are possible in young mothers, not just in older ones. Therefore, NIPT is recommended for all pregnant women who want to exclude risks.
Advantages and limitations of NIPT
NIPT has no contraindications and is performed at the mother's request. Basic or extended options help to rule out fatal malformations and make sure that the baby is okay. The test is performed early. At this time, termination of pregnancy is still relatively safe if the fetus is not viable.
The NIPT procedure takes up to two weeks. The results are discussed during a consultation with a geneticist, and based on them, the woman decides whether to continue or terminate the pregnancy.
In some situations, NIPT is not advisable. If foreign DNA is already present in the mother's blood, it is problematic to isolate the child's DNA. This occurs when:
multiple pregnancy;
bone marrow transplant;
blood transfusion;
using donor eggs.
A pleasant bonus of NIPT is the ability to find out the baby's gender even before the signs that are visible on ultrasound appear. You can also find out his blood type and Rh factor. The test cannot provide any other information about the child's appearance.
Diagnostic testing
What does diagnostic testing include?
This is a double or triple test that is done at 11-13 weeks. Combined diagnostic testing consists of an ultrasound scan and a biochemical blood test.
At 12 weeks, the indicators are most informative. Thanks to numerous studies, mathematical data on healthy embryos and embryos with deviations have been collected. By comparing the values with the norm, conclusions are made about the baby's health. Additional parameters are taken into account: the mother's age and weight, bad habits. The exact obstetric period of pregnancy is also important. It is counted from the first day of the last menstruation.
Using reflected sound waves, ultrasound measures the thickness of the collar space. This is the name given to the accumulation of subcutaneous fluid on the back of the neck. It is an important marker of genetic diseases. The biochemical test of venous blood is aimed at the following indicators:
β-hCG (human chorionic hormone);
PAPP-A is a protein enzyme.
Based on the results of the triple test, the risks of chromosomal abnormalities are calculated: Down syndrome, Edwards syndrome, Patau syndrome. These are defects of chromosomes 21, 13 and 18, respectively. The accuracy of the assessment reaches 70-80 percent.
Advantages and limitations of diagnostic testing
Diagnostic testing is recommended for all mothers. It is simple and safe, and provides a lot of useful information. With the help of tests and ultrasound, the doctor monitors the correct development of the embryo. During ultrasound, the doctor determines:
number of fruits;
skull and brain size;
development of the kidneys, intestines and other internal organs;
the size and shape of the skeletal bones;
placental-uterine blood flow;
state of the chorion;
amount of amniotic fluid.
For accurate measurements, it is important that the ultrasound machine has a high resolution. Then measurements are taken with an accuracy of up to 0.1 millimeters. Based on the diagnostic data, the doctor assesses the risk of miscarriage, malformations, and complications of pregnancy. A blood test helps to clarify the information obtained by ultrasound.
The main limitation of the method is the importance of performing the study at a certain time. Up to 12 weeks, all embryos develop similarly. Therefore, their parameters can be compared with each other and conclusions can be made about deviations from the norm. At later stages, the test will not be informative (ultrasound screening is performed up to 13 weeks 6 days).
Diagnostic testing sometimes gives false positive results. In such cases, the doctor refers the woman for additional examinations - invasive tests. Their results often do not confirm the presence of a developmental anomaly. Invasive interventions are associated with some risk. Therefore, they are prescribed when absolutely necessary due to a combination of other factors. For example, the woman's age and cases of hereditary diseases in the family are taken into account.
Comparison of NIPT and diagnostic testing
Similarities and Differences Between NIPT and Diagnostic Testing
NIPT and diagnostic testing are performed at the end of the first trimester to rule out major genetic abnormalities. But NIPT can be performed earlier, after the 9th week. It is a more accurate method, with errors of about one percent.
Diagnostic testing identifies diseases by indirect signs. For example, by the concentration of hCG in the blood, measurements of the collar space, visualization of the nasal bone. NIPT is a direct method in which the number of defective chromosomes is directly counted.
Which type of testing to choose: selection criteria
A factor worth paying attention to is the risk of abnormalities assessed by a geneticist. If a woman or her relatives have already given birth to children with complex diagnoses, it is worth checking the fetus as early as possible. That is, it is necessary to choose NIPT, since it is performed at 10 weeks.
The basis for NIPT is often solely the woman's desire. If it is important for the mother to know that the baby is developing normally, it is worth signing up for the procedure and stop worrying in vain.
Diagnostic testing has an advantage over NIPT. In addition to excluding pathologies, it helps monitor the pregnancy process itself. Screening assesses the risks of preeclampsia and other complications. That is, the studies are not equivalent and complement each other. Together, they provide a complete picture of the health of the mother and child. In case of twin pregnancy, when one fetus does not develop, it is not advisable to do NIPT. An accurate result will not be obtained if there are three or more fetuses. Then parents should opt for diagnostic testing.
With age, the likelihood of certain pathologies increases due to the accumulation of mutations in germ cells. Therefore, geneticists recommend that pregnant women over 35 years of age not refuse prenatal diagnostics.
Limitations of Prenatal Genetic Testing
What Prenatal Testing Can't Detect
Prenatal genetic testing is aimed primarily at identifying chromosomal abnormalities. Not all of those studied by science, but the most common ones. Diseases of internal organs, allergies and other pathologies cannot be determined in advance.
Such a serious pathology as neural tube defect is diagnosed only in the second trimester. The biochemical marker of this anomaly is not determined by blood analysis at 12 weeks or by NIPT. The good news is that the disease can be avoided. When preparing for conception, experts advise taking folic acid, giving up alcohol and cigarettes.
Ethical Issues Associated with Prenatal Genetic Testing
Any diagnostic procedure is performed voluntarily, with the mother's consent. If parents are ready to accept a child with diseases, they may not do NIPT and other tests.
The doctor's duty is to inform about possible risks and consequences. It is worth remembering that diagnostic testing cannot be performed later than a certain period. If a woman has previously refused it, she will not be able to change her mind.
If NIPT or another test is positive, parents have to make a difficult decision. Genetic abnormalities are incurable at the current stage of medical development. Parents are given the right to decide whether to terminate or keep the pregnancy. Information about the disease in the second case will give parents time to prepare: learn more about child care, find support groups.
Conclusion
NIPT is a safe way to make sure that the future child is healthy and has no critical abnormalities. It is worth doing even if the parents are not sick. No one is immune from risk. Some pathologies are passed on through several generations, while others arise due to random mutations.
Experts recommend doing NIPT for mothers over 35, as well as to find the cause of miscarriages and habitual miscarriages. The test has proven accuracy. A negative result allows you to stop worrying about the baby developing normally and focus on happy anticipation.
