Preimplantation genetic testing (PGT) is an optional procedure performed exclusively during In Vitro Fertilization (IVF) to analyze the genetic makeup of an embryo before transfer. The process involves removing a few cells from the outer layer of a developing embryo and analyzing the DNA. This testing aims to reduce the risk of transferring an embryo with a specific genetic condition or chromosome abnormality, thereby improving the chances of a successful pregnancy. PGT is a collective term for several types of genetic analysis, each focused on a different category of genetic material.
The Different Purposes of Embryo Testing
The decision to perform PGT is driven by three distinct medical rationales, each corresponding to a specific type of test.
PGT-A (Aneuploidy)
Preimplantation Genetic Testing for Aneuploidy (PGT-A) is the most common form, screening for an incorrect number of chromosomes (aneuploidy). Humans normally have 46 chromosomes, arranged in 23 pairs. Embryos with missing or extra chromosomes, such as the extra copy causing Down syndrome, are more likely to result in failed implantation or miscarriage. PGT-A is often recommended for individuals with advanced maternal age, a history of recurrent miscarriages, or multiple failed IVF cycles.
PGT-M (Monogenic Disorders)
Preimplantation Genetic Testing for Monogenic Disorders (PGT-M) is designed for couples who are known carriers of a specific single-gene defect. This test identifies embryos that have or are carriers for conditions caused by a mutation in a single gene, such as Cystic Fibrosis or Huntington’s disease. Before the IVF cycle, a specialized laboratory typically spends about three months building a custom test specific to the known family mutation. PGT-M significantly reduces the chance of passing on an inherited condition by selecting embryos that are unaffected or are healthy carriers.
PGT-SR (Structural Rearrangements)
Preimplantation Genetic Testing for Structural Rearrangements (PGT-SR) is utilized when a parent has a structural abnormality in their chromosomes, such as a translocation or inversion. A translocation occurs when a piece of one chromosome breaks off and attaches to another, potentially leading to an unbalanced amount of genetic material in the embryo. PGT-SR helps select embryos that have a balanced or normal chromosomal structure, which reduces the risk of recurrent pregnancy loss associated with these rearrangements.
The Step-by-Step Testing Process
The physical process of performing PGT begins once the embryo reaches the blastocyst stage, typically on Day 5 or Day 6 after fertilization. At this stage, the embryo has differentiated into two cell types: the inner cell mass (ICM), which forms the fetus, and the trophectoderm (TE), which forms the placenta and surrounding membranes. The blastocyst stage is considered more resilient to the manipulation required for testing.
A specialized embryologist then performs an embryo biopsy, involving the removal of a small number of trophectoderm cells, typically five to ten cells. This is accomplished using a laser to create an opening in the outer shell of the embryo, known as the zona pellucida, and a fine pipette to separate and remove the cells. The inner cell mass must be left completely untouched to minimize the risk of harm to the future fetus.
Immediately following the biopsy, the embryo is cryopreserved while the extracted cells are prepared for analysis. The trophectoderm cells, which contain the embryo’s DNA, are shipped to a specialized genetics laboratory. The laboratory uses advanced techniques like Next-Generation Sequencing (NGS) to analyze the genetic material. Results typically become available within one to two weeks, during which time the embryo remains frozen awaiting the diagnosis before a transfer can be scheduled.
Interpreting Results and Mosaicism
The analysis of the trophectoderm cells yields one of three primary result categories: euploid, aneuploid, or mosaic. A euploid result indicates the sample has the correct number of 46 chromosomes, suggesting a normal chromosomal complement and a higher likelihood of successful implantation and live birth. Conversely, an aneuploid result signifies an abnormal number of chromosomes, such as a missing or extra copy. These embryos are generally not recommended for transfer due to high risks of miscarriage or developmental conditions.
Mosaicism represents the most complex and debated result, occurring when the embryo contains two or more populations of cells with different genetic compositions. This means some cells in the trophectoderm biopsy are euploid while others are aneuploid. The diagnosis is inferred based on the ratio of normal to abnormal cells found in the sample. The level of mosaicism is reported as a percentage, and laboratories use varied thresholds to define an embryo as low-level or high-level mosaic.
Mosaicism complicates clinical decision-making because the biopsied trophectoderm cells may not perfectly reflect the genetic makeup of the inner cell mass (the future fetus). While euploid embryos have higher implantation rates, mosaic embryos can still result in a healthy pregnancy and live birth, though at lower rates. Transferring a mosaic embryo, often considered when no euploid embryos are available, is typically approached with caution and requires close monitoring during the resulting pregnancy.
Accuracy and Current Limitations
PGT is an accurate procedure, but it is not infallible, and its reliability is constrained by the sampling process. The primary limitation is that PGT analyzes only a small number of trophectoderm cells, which may not perfectly represent the genetics of the inner cell mass due to mosaicism. This sampling limitation introduces a small risk of a false negative result (incorrectly labeling an embryo as euploid) or a false positive result (incorrectly discarding an aneuploid embryo).
The technique also carries a small technical risk to the embryo, though it is considered minimal and safe in experienced hands. The physical process of the biopsy, freezing, and thawing results in a small chance of damage that could prevent implantation. Since PGT is a screening test, any resulting pregnancy still requires follow-up verification. Clinical guidelines often recommend that patients undergo subsequent prenatal diagnostic testing, such as amniocentesis or chorionic villus sampling (CVS), to confirm the genetic status of the fetus.