Preimplantation Genetic Testing (PGT) 101

What is Preimplantation Genetic Testing (PGT)?

Preimplantation genetic testing (PGT) is a procedure in the in vitro fertilization (IVF) process whereby embryos created through IVF are tested for chromosomal abnormalities and gene disorders before being placed into the uterus. The use of PGT among IVF patients in the U.S. has been steadily increasing in the past few years and is now utilized in around 40-50% of IVF procedures at AFCC.

What are the Types of PGT?

• Preimplantation Genetic Testing for Aneuploidy (PGT-A): Aneuploidy (chromosomal abnormality) is a common cause of failed implantation and miscarriage. PGT-A analyzes for an abnormal number of chromosomes and abnormal structure. Examples of aneuploidy are a missing chromosome (monosomy) or an extra chromosome (trisomy). Selecting a chromosomally normal embryo for transfer helps improve the chances of a successful pregnancy.
• Preimplantation Genetic Testing for a Monogenic Disease (PGT-M): PGT-M identifies specific gene mutations that one or both parents are known to carry, allowing unaffected embryos to be transferred, reducing the chance of severe genetic disease in the offspring. Examples of genes tested by PGT-M include cystic fibrosis, sickle cell anemia, and muscular dystrophy.
• PGT for Chromosome Structural Rearrangement (PGT-SR): PGT-SR is used for patients with known chromosomal structural rearrangements, such as inversion or translocation. These rearrangements can lead to embryos with incorrect chromosomal material, often resulting in repeated miscarriages. PGT-SR helps select embryos with the correct chromosomal structure to increase the chances of a live birth.

How is PGT Performed?

After eggs are retrieved and fertilized in the IVF lab (usually after 5-7 days of development), an embryologist performs an embryo biopsy by removing a few cells from the part of the embryo that will become the placenta. These cells are then sent to a specialized genetics lab for analysis.

What are the Indications for Performing PGT?

• Advanced Reproductive Age: Miscarriages and chromosomal abnormalities are more common as the female partner's age increases. PGT-A is often recommended for women older than 35 undergoing IVF, as it may decrease miscarriages and increase live birth rates for this age group.
• Single Gene Defects: In couples affected by or carrying a genetic disease due to single gene abnormalities (e.g., cystic fibrosis, muscular dystrophy), PGT-M can identify embryos carrying the disease, thus avoiding its transmission. For diseases transmitted through the X or Y chromosome, selecting an unaffected gender can also ensure a disease-free child.
• Reducing Multiple Births: By selecting a single healthy embryo through PGT-A, individuals and couples can avoid transferring multiple embryos, thus reducing the likelihood of multiple births and associated health risks for both mother and babies.
• Reducing Recurrent Miscarriages: Selecting a chromosomally normal embryo using PGT-A may decrease miscarriages in couples with chromosomal abnormalities that contribute to recurrent miscarriages.

Are There Any Concerns with PGT?

• The value of PGT-A as a universal screening test for all IVF patients has not been demonstrated.
• PGT-A's role in recurrent pregnancy loss and implantation failure remains unclear.
• PGT-A results may be inconclusive, leading to potential non-transfer of embryos that might otherwise develop into healthy babies.
• PGT-A adds additional expense and requires specialized expertise from lab personnel.
• Embryos can be damaged during the biopsy process, though this risk is minimized by experienced embryologists.
• Even with advanced PGT technology, the error rate may be as high as 2%.

Preimplantation genetic testing offers many potential benefits for certain IVF patients. However, understanding both the indications and limitations of PGT is crucial to making informed decisions about its use in your fertility journey.