PGT & Genetics
Preimplantation genetic testing (PGT) is an advanced fertility procedure in which embryos are screened for chromosomal abnormalities and genetic disorders. The test is performed on embryos created through in vitro fertilization (IVF) before they are transferred to the recipient’s uterus. There are three different types of PGT available:
- Preimplantation genetic testing for aneuploidies (PGT-A), formerly known as preimplantation genetic screening (PGS) and comprehensive chromosomal screening (CCS)
- Preimplantation genetic testing for monogenic disorders (PGT-M), formerly known as single-gene preimplantation genetic diagnosis (PGD)
- Preimplantation genetic testing for structural chromosomal rearrangements (PGT-SR)
Why Is PGT Recommended?
Many patients who are undergoing IVF choose to use PGT for peace of mind. By identifying and selecting genetically healthy embryos, PGT helps reduce the risk of having a miscarriage, thus giving aspiring parents a better chance of having a healthy pregnancy, delivery, and baby. PGT is usually recommended in specific circumstances, including:
- Cases of recurrent pregnancy loss
- Multiple unsuccessful IVF cycles
- A family history of genetic or chromosomal abnormalities
- Advanced maternal age
PGT technology is also sometimes used for family balancing and sex selection. This can be particularly useful for patients who have hereditary conditions that are gender-specific.
How Is PGT Testing Done?
No matter which type of PGT testing you have, the process is very similar.
First, eggs are retrieved from the female patient (or donor) and fertilized with sperm from the male patient (or donor), forming a single-celled zygote. Over the next few days, the zygote transforms from a single cell to an embryo consisting of 16 cells. The embryo is monitored and allowed to develop for five to six days so that it can reach the blastocyst stage. At this point, the embryo consists of approximately 200-300 rapidly dividing cells.
Structurally, a blastocyst embryo consists of two types of cells:
- The inner cell mass, which will eventually develop into the fetus as well as the amniotic sac
- The trophoblast, which provides the embryo with nourishment and later develops into the placenta
Once the embryo has reached the blastocyst stage, a few cells are biopsied from the outer trophoblast layer of the embryo. During this time, the embryo is frozen until the genetic testing has been completed.
Types of PGT, Explained
Each of the different types of PGT is used to test specific types of genetic disorders.
PGT for Aneuploidies (PGT-A)
Preimplantation genetic testing for aneuploidies (PGT-A) is used to identify embryos that have an irregular number of chromosomes. Contained within each and every cell nucleus, chromosomes are like living blueprints that contain all the information that cells need to create a body. Every living thing, both animal and plant, has a set number of chromosomes. Humans have 23 pairs of chromosomes, with one set of chromosomes coming from our mothers and another set coming from our fathers, for a total of 46 individual chromosomes.
Because chromosomes contain all the information each cell needs to replicate, chromosomes play a critical role in the cell division process. When an error occurs during cell division, it can lead to a condition known as aneuploidy, which is when the cells have too many or too few chromosomes. Aneuploidy is a common cause of many genetic disorders, including Down syndrome, Klinefelter syndrome, and Edwards syndrome. Children who have these disorders are often born with certain anomalies, such as delayed development, musculoskeletal irregularities, intellectual disabilities, and specific physical characteristics.
PGT-A helps reduce the risk of having a child with aneuploidy disorders by counting the number of chromosomes contained within the cells and identifying embryos with the correct number of chromosomes. This procedure is often recommended for cases of recurrent pregnancy loss and advanced maternal age.
PGT for Monogenic Disorders (PGT-M)
Preimplantation genetic testing for monogenic disorders (PGT-M) is used to identify embryos that have specific genetic disorders. This test is typically recommended for patients who have a known family history of genetic conditions. Before the embryos are created, the patient(s) must meet with a genetic counselor for an expanded carrier screening to see which, if any, genetic disorders they are at risk for passing on to their child. The genetic information gathered from this screening will determine which genetic conditions the embryos will be tested for. This process can take several months to complete.
By analyzing the embryos for genetic abnormalities and only using the healthiest embryos for transfer, the risk of passing on a genetic disorder is significantly reduced.
PGT for Structural Chromosomal Rearrangement (PGT-SR)
Preimplantation genetic testing for structural chromosomal rearrangement (PGT-SR) is used to analyze embryos for certain types of chromosomal abnormalities, specifically chromosomal deletions, duplications, and rearrangements. In these cases, the embryos have the correct number of chromosomes, but the chromosomes themselves have some sort of structural abnormality. Although these abnormalities usually don’t cause any apparent abnormalities in carriers, they increase the risk of infertility, recurrent pregnancy loss, and the likelihood of having a child with genetic abnormalities.
PGT-SR helps to screen for translocations so that only the healthiest embryos are selected for transfer. This type of genetic test is typically only recommended for patients who are carriers of certain types of structural rearrangements. In many cases, patients may not be aware that they are carriers of structural rearrangements unless they have gone through genetic counseling or testing.