Pre-implantation genetic testing (PGT) is a group of genetic tests performed on embryos created through IVF before they are transferred to the uterus. Following ovarian stimulation, IVF involves the fertilization of eggs in the laboratory, and the resulting embryos grow for about 5 to 7 days until they reach the "blastocyst" stage. At that point, a small number of cells (about 5–10) are carefully removed from the outer layer of the embryo (called the trophectoderm, which later becomes the placenta) and sent for genetic analysis while the tested embryos are frozen awaiting results.
There are three main types of PGT:
1. PGT-A (aneuploidy)
This test checks whether embryos have the correct number of chromosomes, i.e., 23 pairs (one set from the egg and one set from the sperm) for 46 total, with one pair of sex chromosomes - XX (female) and XY (male). An abnormal number of chromosomes is a common cause of miscarriage and failed implantation. For example, Down Syndrome is associated with three copies of chromosome number 21, instead of two.
It’s important to note that PGT-A is not recommended for everyone undergoing IVF. Both the American Society for Reproductive Medicine (ASRM) and the American College of Obstetricians and Gynecologists (ACOG) state that there is not enough evidence to support its routine use for all patients.
Research shows that while PGT-A can help identify which embryo to transfer first, it does not increase the overall chance of having a baby from a single IVF or egg freezing cycle for most people, particularly women under age 38. Instead, its main benefit may be shortening the time to pregnancy by helping prioritize embryos for transfer.
It's also important to know that PGT-A is not a perfect test. The biopsy samples cells that will become the placenta, not the baby itself, so the results may not always reflect the embryo as a whole. Some embryos labeled as abnormal or mosaic (containing a mix of normal and abnormal cells) can still result in healthy pregnancies and babies.
For these reasons, PGT-A is generally recommended only in certain situations, such as for people of advanced reproductive age or those with several blastocyst-stage embryos. Your fertility specialist can help determine whether PGT-A is likely to be beneficial based on your individual circumstances.
2. PGT-M (monogenic)
This tests for a specific inherited genetic condition that runs in the family, such as sickle cell disease or cystic fibrosis. Genes are found on chromosomes, which hold all of a person's DNA, containing roughly 20,000–25,000 genes. PGT-M tests for disorders which are caused by a specific gene, such as Huntington’s disease, Tay-Sach’s disease, and Hemophilia.
3. PGT-SR (structural rearrangements)
This test looks for chromosome structural problems like translocations that represent a rearrangement of the normal number of chromosomes. This variation can lead to infertility and recurrent miscarriage.
Only embryos that pass the PGT test are selected for transfer. Regardless of the age of the woman, ASRM strongly recommends a single embryo transfer of a normal PGT embryo. It is important to understand that PGT is not perfect. In up to five percent of tests, the result may report an "abnormal" embryo but is incorrect and may have led to a live birth. Confirmation testing during pregnancy, such as amniocentesis is still recommended during pregnancy even after PGT.
Can genetic testing be done on eggs alone (without creating embryos)?
Genetic testing on eggs alone is not routinely performed as the current approaches have many limitations. PGT, which is done on embryos, cannot be performed on unfertilized eggs. The only form of genetic testing that can be done at the egg stage is called polar body biopsy, which analyzes small byproducts released by the egg during its maturation. However, this approach has significant limitations and is not widely used for the following reasons:
It only detects maternal (egg) genetic problems, not paternal (sperm).
It can miss a significant percentage of chromosome errors, including those that arise after fertilization.
It may overdiagnose abnormalities.
It is more expensive (both polar bodies must be tested) and technically challenging.
It has shown limited impact on improving live birth rates.
For these reasons, the standard and most reliable approach is to use embryos at the blastocyst stage, which provides information about both egg and sperm genetics and has been shown to be safe for the embryo. PGT requires creating embryos through IVF - it cannot be done on frozen eggs alone without first fertilizing them. While PGT can be a helpful tool for some people undergoing IVF, it is not always necessary or appropriate for everyone. Your fertility specialist can help you understand the benefits and limitations of each type of PGT and determine whether genetic testing is the right choice based on your medical history, family history, and reproductive goals.
Dr. Mark Trolice is the Director of The IVF Center in Winter Park, Florida, and a Professor of Obstetrics and Gynecology at the University of Central Florida College of Medicine. A double board-certified reproductive endocrinologist and infertility specialist, Dr. Trolice is a nationally recognized leader in fertility care with decades of experience in patient care, research, and education. He is passionate about improving access to fertility care and educating patients through his podcast, publications, and community outreach.
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