Preimplantation Genetic Testing, or PGT, is a specialized laboratory procedure performed in conjunction with in vitro fertilization (IVF) to evaluate the genetic material of an embryo before it is transferred into the uterus. This testing allows fertility specialists to screen for chromosomal abnormalities or specific inherited genetic disorders. The primary goal is to help identify embryos with the highest likelihood of resulting in a successful pregnancy and the birth of a healthy child. PGT offers an opportunity to gain information about an embryo’s genetic makeup that is not possible through visual assessment alone.
The Different Categories of PGT
The umbrella term PGT encompasses three distinct classifications, each designed to screen for different types of genetic issues within the embryo. The most frequently performed type is PGT-A, which stands for Preimplantation Genetic Testing for Aneuploidy. Aneuploidy describes an abnormal number of chromosomes, meaning an embryo has more or fewer than the standard 46 chromosomes. PGT-A screens all 24 chromosomes—the 22 non-sex chromosomes and the X and Y sex chromosomes—to select embryos that are euploid, or chromosomally normal. This test is commonly utilized because chromosomal errors are a major reason for implantation failure and miscarriage.
Another category is PGT-M, which is Preimplantation Genetic Testing for Monogenic Disorders. This test is used when there is a known risk of passing on a condition caused by a mutation in a single gene, such as Cystic Fibrosis, Huntington’s Disease, or Sickle Cell Anemia. Unlike PGT-A, PGT-M is highly specific and requires a custom test to be developed for the family’s particular mutation before the IVF cycle begins. The goal is to identify embryos that are unaffected by the specific inherited condition.
The third classification is PGT-SR, or Preimplantation Genetic Testing for Structural Rearrangements. This test is designed for couples where one partner is known to carry a balanced chromosomal rearrangement, such as a translocation or an inversion. Although the carrier parent is typically healthy because they have the correct total amount of genetic material, they are at an increased risk of creating embryos with an unbalanced amount of chromosomal material. PGT-SR screens the embryos to find those with a balanced or normal chromosome arrangement, thereby reducing the risk of miscarriage or a child born with a resulting genetic condition.
The Procedure for Embryo Testing
The PGT process begins after the eggs have been retrieved and fertilized through IVF, and the resulting embryos are cultured in the laboratory. For testing to occur, the embryos must be allowed to develop to the blastocyst stage, which is typically reached on day five or day six following fertilization. The blastocyst has differentiated into two distinct cell groups: the inner cell mass, which will form the fetus, and the trophectoderm, which will form the placenta.
The actual PGT procedure involves a delicate process called an embryo biopsy, which is performed by a trained embryologist. Using specialized micromanipulation tools, a small number of cells, usually between five and ten, are carefully removed from the trophectoderm layer of the blastocyst. This process leaves the inner cell mass undisturbed and is performed with minimal risk to the embryo’s future development. The biopsied cells are then prepared and sent to a specialized genetics laboratory for analysis.
While the genetic testing is underway, which can take anywhere from a few days to a couple of weeks, the biopsied embryos are cryopreserved, or flash-frozen, using a technique called vitrification. The laboratory analysis uses advanced genetic sequencing or microarray technology to examine the DNA from the biopsied cells, looking for the specific chromosomal counts or gene mutations relevant to the PGT type being performed. Once the results are available, classifying the embryos as euploid (normal), aneuploid (abnormal), or sometimes inconclusive, a frozen-thawed embryo transfer is scheduled for the selected genetically sound embryo.
Who Benefits from Preimplantation Testing
PGT is an option for many individuals and couples undergoing IVF, but it is particularly beneficial for those with specific clinical indications. Advanced maternal age is a primary reason for seeking PGT-A, as the chance of an embryo having an abnormal number of chromosomes increases for women over the age of 35. Screening for aneuploidy is also frequently recommended for couples with a history of recurrent pregnancy loss or multiple cycles of IVF that have previously failed to result in a pregnancy.
Patients who are known carriers of a specific genetic disorder are candidates for PGT-M. This includes couples where both partners carry a recessive condition, or a female partner carries an X-linked condition, allowing them to proactively reduce the chance of passing the condition to their child. PGT-SR is indicated for individuals who have had a karyotype test revealing a structural chromosome rearrangement. This testing helps these carriers avoid the high risk of miscarriage and congenital anomalies associated with unbalanced chromosomes.