What Is Genetic Testing?
Genetic testing involves examining the DNA of the parents-to-be for information about whether their baby has certain genetic disorders. Many genetic disorders can be diagnosed before birth. The health care professionals may recommend genetic testing during pregnancy if any of the parents have a family history of genetic disorders. Genetic screening can be chosen by the couples who had a fetus or baby with genetic disorders.
Genetic testing involves checking for changes in the following:
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Genes - DNA sequences help to study and identify variations in genes that can be a cause of genetic disorders.
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Chromosomes -The long lengths of DNA or whole chromosomes are analyzed to check if there are large genetic changes, like an extra copy of a chromosome which can be a cause of a genetic condition.
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Proteins - The amount or activity level of enzymes or proteins are done in biochemical genetic tests. The abnormalities can indicate a genetic disorder.
What Are the Types of Genetic Testing During Pregnancy?
The genetic testing involved during pregnancy are:
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Screening Tests - The screening tests help in identifying if the baby is more or less likely to have birth defects and genetic disorders. These tests include prenatal cell-free DNA screening, blood tests, and a specific type of ultrasound. These tests are usually done during the first or second trimester. Prenatal screening tests can not make a definitive diagnosis so if the results indicate a high risk for a genetic disorder, the doctors will discuss the options for confirmation of the diagnosis like a diagnostic test.
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Diagnostic Tests - If a screening test indicates any genetic disorders or if the family history, medical history, or age has an increased risk of developing a baby with a genetic disorder, an invasive prenatal diagnostic test may be recommended. A diagnosis can be confirmed by diagnostic tests.
Who Should Undergo Genetic Testing?
All pregnant women, regardless of risk level or age, are recommended for first-trimester screening. Besides this, genetic testing is usually offered to couples with a family history of certain conditions, or a higher risk is identified for a baby with a genetic health condition or birth defect. Whether to undergo genetic testing is a personal decision of the couple.
How Is Genetic Testing Performed?
Genetic testing is performed by collecting samples of blood, hair, or tissue. In pregnant women, a sample of amniotic fluid is taken or a tissue sample from the placenta is collected.
What Are the Genetic Screening Tests Available During Pregnancy?
Screening Test in the First Trimester -
During the first trimester of pregnancy, the doctor might suggest genetic testing if there is a risk of a medical issue. The tests include:
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Sequential Screen - This tests both the ultrasound and the blood tests to look for the risk of Edwards syndrome (trisomy 18), Down syndrome (trisomy 21), Patau syndrome (trisomy 13), and brain and spine problems. The results will be provided between 10 and 13 weeks of pregnancy. Depending on the results, further testing may be provided by the doctors.
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Integrated Screening - This test also has both the ultrasound and the blood tests that are used for checking the risk factors of trisomy 18, Down syndrome, and spine and brain problems. The results will be provided in around 12 weeks, and the second part of the tests happens during the second trimester. Integrated screening is more accurate than a sequential screen, but the results may take a longer time.
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Cell-Free Fetal DNA Testing - A small fragment of a baby’s DNA winds up in the mother's blood which helps in determining the chance of chromosomal problems. After 10 weeks, the doctor collects a sample from the mother and performs laboratory tests for signs of trisomy 18, trisomy 13, down syndrome, and problems with sex chromosomes. If any of the screening tests show a risk of a birth defect, the diagnostic tests for confirmation will be suggested by the doctors. The problems with the brain or spine are not checked by the cell-free DNA test, so it is advisable for another blood test in the second trimester to detect those problems.
Screening Test in the Second Trimester -
During the second trimester of pregnancy, the doctor will offer some typical tests that include:
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Integrated Screening (Part 2) - If the first part of the integrated screening is done in the first trimester, another blood test is recommended between 16 weeks and 18 weeks of pregnancy.
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Maternal Serum Quad Screen - This blood test is used to detect proteins in the blood which can say if the baby has a risk of trisomy 18, down syndrome, and spine or brain problems. This test can be performed between 15 weeks and 21 weeks.
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AFP (Alpha-Fetoprotein) Test - In this test, screening for neural tube defects is done. It is performed when the patient has done the cell-free DNA test, which does not screen for defects.
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Ultrasound - Around week 20, an ultrasound is performed to look for birth defects like heart problems, kidney problems, and cleft palate.
What Are the Diagnostic Tests Carried Out to Check the Genetic Problems?
There are two common diagnostic tests that are chorionic villus sampling and amniocentesis. Both tests are invasive and involve taking cells from the fetus and analyzing the cells under a microscope. This helps in determining whether the chromosomes are damaged or the fetus has few or many chromosomes present which could result in a genetic problem.
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Chorionic Villus Sampling (CVS) - This test is done during the first trimester of pregnancy normally at 10 to 12 weeks. CVS tests involve collecting a small part of the placenta in the uterus. The doctor collects the tissue either with a needle through the mother's stomach or using a thin tube into the cervix. There can be cramps, infection, or bleeding for some women. This test cannot identify neural tube defects.
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Amniocentesis - In this test the doctor puts a thin, long needle through the mother's stomach into the sac that surrounds the fetus to take out a small amount of fluid. The proteins and cells in the amniotic fluid are then checked in a laboratory. The test is performed between 15 weeks and 20 weeks of pregnancy. Amniocentesis can cause cramping, bleeding, or infection.
What Does the Result of Genetic Testing Convey?
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If the Test Is Positive - That means that the baby is at higher risk of having the disorder when compared with the normal population. It does not mean that the baby definitely has the disorder.
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If the Tests Are Negative - That means that the baby is at a lower risk of having the disorder when compared to the normal population. It does not rule out the possibility that the baby has the disorder.
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Diagnostic Tests Results - Diagnostic testing provides a more definite result and is an option for all pregnant women.
The doctors and the genetic counselor will discuss what the screening test results mean and help in deciding the next steps.
Conclusion:
Each woman and each pregnancy is different, and if family history or doctors detect certain illnesses, genetic abnormalities can occur. Choosing to have carrier screening and diagnostic testing is a personal choice. If a parent is considering prenatal genetic screening, it is good to have counseling with a genetic counselor who will help in guiding through the testing process and discussing the results.
