What is Preimplantation Genetic Testing for Aneuploidy (PGT-A) and how is it useful?
For patients considering the option of in vitro fertilization (IVF), it’s likely that your physician will discuss the option of Preimplantation Genetic Testing for Aneuploidy (PGT-A) as part of your treatment plan. PGT-A is a form of genetic testing used to look for chromosome abnormalities (also known as “aneuploidy”) in embryos before they are transferred into the uterus to attempt pregnancy. Formerly known as Preimplantation Genetic Screening (or PGS), PGT-A has been available for over a decade now but previously was only used for patients with specific infertility indications, such as recurrent pregnancy loss. However, in the past several years, advances in both embryo culture and chromosome screening technology have made it possible for PGT-A to be performed safely as a routine part of the IVF process.
What are Chromosomes?
Chromosomes are the structures inside our cells that hold all our genes—the information that tells our bodies how to work properly. Humans have 23 pairs of chromosomes, for a total of 46 chromosomes, and receive one set of chromosomes from each parent - 23 chromosomes in the oocyte (egg) cell, and 23 in the sperm cell. However, when our bodies create eggs and sperm random errors can occur that cause our egg/sperm cells to have extra or missing chromosomes - a phenomenon also known as ‘aneuploidy’.
Our bodies are very particular about needing exactly 46 chromosomes in 23 pairs, so having any extra or missing chromosomes is usually not compatible with healthy human development. If pregnancy is attempted with a chromosomally abnormal embryo, typically that embryo will either fail to implant in the uterus (resulting in no pregnancy that month) or implant but then result in a pregnancy loss. Beyond that, certain chromosome abnormalities can result in a live birth with physical or intellectual disabilities, the most common being Down Syndrome (which occurs when there are three copies of chromosome 21, instead of two).
How Is PGT-A Useful?
Chromosome abnormalities occur more frequently in eggs cells as a female ages, with the percentage of chromosomally abnormal embryos increasing dramatically after age 37. However, even embryos from women in their twenties and early thirties have a significant chance of chromosome abnormalities. Whether an embryo is normal (euploid) or abnormal (aneuploid) is not outwardly visible when observing the embryos in the IVF laboratory, so without PGT-A it is not possible to know whether the embryo being transferred has the ability to become a healthy, chromosomally normal pregnancy. The use of PGT-A helps identify healthy embryos for transfer and therefore significantly reduces the risk of pregnancy loss, failed implantation, or a chromosomally abnormal pregnancy in comparison to treatment without PGT-A.
Like any form of testing, PGT-A has limitations which are important to keep in mind. While detection rates are high (approximately 98-99%) they are not 100%, and there are small but present risks of both false positive and false negative results. In addition, certain rare types of chromosome abnormalities may not be detected. Because of the limitations of PGT-A, follow up prenatal testing should still be considered and further discussion of prenatal testing options with a prenatal genetic counselor and/or OB/GYN is recommended. In addition, PGT-A cannot screen for single gene disorders or conditions such as autism, cerebral palsy, or sporadic birth defects (which in many cases do not have a specific, identifiable genetic cause).
Despite these limitations, PGT-A offers the significant benefit of increasing the likelihood of a healthy pregnancy after embryo transfer while at the same time decreasing the chance of pregnancy loss—helping patients achieve their family-building goals with fewer setbacks along the way.