During a genetic consultation, you’ll hear many acronyms, and they can get confusing. As you aim to have a healthy baby, you’ll evaluate different screening options to find what’s best for you. Recently, some terminology in the world of assisted reproductive technology have changed, so what you were used to hearing may go by a different name. Let’s take a closer look at two embryo screening options and what they mean: PGT-M vs. PGT-A.
PGT-M vs. PGT-A by Definition
Preimplantation genetic testing (PGT) refers to screening tests that embryos may undergo to look for a heritable genetic mutation or chromosome abnormality, which could lead to your child having a debilitating disorder or disease during childhood or later in life. What type of issue you’re seeking to identify determines what type of testing you undergo.
PGT for monogenic disorders (PGT-M) determines whether you have a heritable genetic mutation that can be passed on to your children. PGT-M was previously called preimplantation genetic diagnosis (PGD). This test looks for diseases caused by a mutation in a single gene, and it is offered to families with a known genetic mutation. That risk may have been discovered through routine genetic screening, such as carrier screening, or because one parent or family member already has a known genetic diagnosis.
PGT for aneuploidy (PGT-A) determines whether there are chromosomal abnormalities in your embryos, which can impact the chance of successful embryo implantation. This test was previous called preimplantation genetic screening (PGS). Chromosomes are packages of genes within cells. Extra or missing chromosomes can lead to miscarriage or a chromosomal syndrome like Down syndrome. These abnormalities happen randomly during embryo development and are not related to inherited genetic risks. PGT-A looks to see whether the embryos have all 46 chromosomes.
Both tests are done in conjunction with in vitro fertilization (IVF), but not everyone needs both tests. Your doctor and genetic counselor can guide you through the differences and which path is right for your family. Both PGT-A and PGT-M are done by removing five to 10 placental cells from the embryos created during IVF. The cells are examined for abnormalities based on which test you’re having, and healthy embryos are preserved for implantation.
Who Should Undergo PGT-A?
Anyone going through IVF can go through PGT-A, and this test is becoming increasingly common among women undergoing IVF. One of the main benefits to getting the test is to increase your chances of transferring an embryo that will implant. For women in their 20s, about 35 percent of embryos tested with PGT-A have a chromosome abnormality. As women age, their chances of having an embryo with a chromosome abnormality that would lead to miscarriage increases.
You may benefit from PGT-A even if you have no family history of an inherited condition and normal results on a genetic carrier screening.
Who Should Undergo PGT-M?
PGT-M is only offered to couples with a possibility of passing on an inherited genetic condition. This may be cystic fibrosis, Tay-Sachs disease, an inherited cancer syndrome or hundreds of other conditions. DNA testing and genetic carrier screening must be done to understand the specific mutation(s) involved and verify that accurate testing can be offered before PGT-M is considered. The PGT-M test is developed based on the specific mutation you want to identify.
You may want to have PGT-A along with PGT-M to ensure that you are transferring an embryo with a higher likelihood of implanting.
Are the Tests Accurate?
Prenatal genetic testing is not a perfect science, and there is a chance for false positives or false negatives. A false positive test result indicates that an embryo is affected or has the mutation when it is actually unaffected or doesn’t carry the mutation. A false negative test result indicates that an embryo is not affected or does carry the mutation, when the embryo actually is affected or does carry the mutation. This is because there’s a limited number of cells available for testing and embryo mosaicism, or embryos with both normal and abnormal cells.
Overall, PGT-A and PGT-M are able to diagnose genetic defects with more than 98 percent accuracy. However, about 3 to 4 percent of babies born have some type of birth defect or genetic condition. PGT-A and PGT-M do not rule out all birth defects or genetic conditions; therefore, you should confirm the results during pregnancy through prenatal screening for chromosome abnormalities or prenatal diagnostic testing via chorionic villus sampling (CVS) or amniocentesis.
A fertility genetic counselor can provide pre-test counseling to be sure you understand the test’s accuracy and limitations, as well as its risks and benefits, to help you decide if PGT-M or PGT-A is right for you.
Is the Testing Safe?
Many couples wonder whether embryo screening is safe. For both tests, the biopsy — removal of cells — happens around day five or six of embryo development. About five to 10 placental cells — the cells that will become the placenta as opposed to fetal cells — are removed for testing by a skilled embryologist. Those cells only need to be taken once, even if you’re doing both PGT-M and PGT-A at the same time.
So far, research and experience has shown that this biopsy is safe.”More than 22,000 embryos have been biopsied at our clinic, and only 10 of these embryos have not survived the biopsy process,” said Alison Coates, PhD, Lab Director at Oregon Reproductive Medicine.
Whether you are considering PGT-A, PGT-M or both, it is important to understand the potential benefits, risks and limitations of these tests. Reviewing this information with a genetic counselor familiar with these tests is the best step toward making an informed decision on what is best for your family.
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Patricia Chaney is a freelance writer specializing in health and wellness. She has more than 10 years of experience demystifying health topics for people and promoting wellness. Her work has appeared on numerous health system websites and in industry journals. Ms. Chaney has a passion for health literacy, natural health, fitness, and food.