Three kinds of embryo genetic tests and what they screen for
PGT testing refers to preimplantation genetic testing. This refers to a group of screening tests used to help identify and reduce the risk of genetic abnormalities in embryos during the in vitro fertilization (IVF) process. In this article, we’ll discuss the three types of PGT and when they can be most appropriate and effective.
Understanding the Genetic Structure of a Standard Embryo
To understand how genetic testing works, we need to define some underlying terms and ideas. Human cells all contain chromosomes, structures that carry the genes which determine our traits and features. A typical embryo contains 46 chromosomes: 23 each from the egg and sperm it developed from. An embryo with this standard chromosomal structure is called a euploid embryo.
What Is Preimplantation Genetic Testing?
Genetic testing can play a role in identifying abnormalities in the embryo and maximizing the chances of a successful IVF pregnancy. Testing the embryos before transfer is called preimplantation genetic testing (PGT). PGT involves embryos at the day 5 or 6 blastocyst stage, when they are each a cluster of cells with a defined outer layer. A technician takes a sample of approximately 10 cells from this outer layer to test. Typically, the embryos are then frozen while the test results are analyzed. Then, the intended parent(s) and their fertility team can use the test results to decide which embryo to transfer.
There are three current categories of PGT: PGT-A, PGT-SR, and PGT-M. Let’s look more closely at each type of PGT.
PGT-A: Preimplantation Genetic Testing for Aneuploidy
(Formerly known as CCS or PGS)
As discussed earlier, typical, or euploid, embryos contain 23 pairs of chromosomes from the sperm and egg. On the other hand, some embryos, called aneuploid, develop with extra or missing chromosomes. Aneuploidy is the most common cause for unsuccessful implantation and pregnancy loss. It can also lead to other conditions or disabilities. For instance, Trisomy 21, or “Down Syndrome,” occurs in aneuploid embryos with an extra copy of chromosome 21. PGT-A looks for missing or extra chromosomes in the embryo to help inform transfer decisions. Keep in mind that this test will also reveal the biological sex of the embryo. If you would prefer to not know that information, let your team know before receiving the results of the test.
PGT-SR: Preimplantation Genetic Testing for Structural Rearrangement
This test also looks for aneuploidy in embryos. In contrast to PGT-A, however, it is most commonly used when the sperm or egg provider has a known structural rearrangement. This occurs when parts of a chromosome are copied, deleted, inverted, or swap positions. Structural rearrangements do not necessarily impact health, and many patients with structural rearrangements are not aware until they receive genetic testing as part of their fertility assessment. However, individuals with structural rearrangements can pass on unbalanced chromosomal abnormalities to embryos that typically result in a miscarriage. PGT-SR helps to reduce this risk.
PGT-M: Preimplantation Genetic Testing for Monogenic Disease
(Formerly known as PGD)
PGT-M primarily exists for patients with a known inherited disorder caused by mutations in a single gene. These monogenic disorders include Huntington’s disease, cystic fibrosis, and BRCA1. PGT-M can help identify and select embryos unaffected by the hereditary condition. PGT-M involves a longer process than the other types of preimplantation genetic testing. Your team will develop a custom map of your and/or your partner’s genetic variations to create a genetic fingerprint to match the embryos against. In some cases, this process may also involve mapping DNA samples from your family members. This mapping process can take several weeks. At the end, the embryos are compared to the custom genetic analysis to determine which, if any, carry the same condition.
PNWF Offers In-House Genetic Counseling
We understand that genetic testing can feel overwhelming, and we offer genetic counseling in our Seattle clinic to help you navigate this complex topic. Keep in mind that everyone’s fertility journey is unique. You may not need to use every form of PGT – or you may choose not to pursue genetic testing at all. We are here to help you understand your options as you choose your best path.
Additional information on PGT is available at RESOLVE.