Preimplantation Genetic Testing

Preimplantation Genetic Testing (PGT)

Preimplantation Genetic Testing (PGT) is a specialized procedure performed alongside in vitro fertilization (IVF). It allows for the screening of embryos for genetic conditions before they are transferred to the uterus. This can help reduce the risk of miscarriage and prevent the transfer of embryos affected by certain genetic disorders.

What Is PGT-A?

PGT for Aneuploidy (PGT-A) checks embryos for the correct number of chromosomes. A typical human cell has 46 chromosomes—23 from each parent. If an egg or sperm has a missing or extra chromosome, the resulting embryo may also have too many or too few, a condition known as aneuploidy.

Many aneuploid embryos do not implant or lead to miscarriage. Others can result in conditions like:

• Down syndrome (Trisomy 21)
• Trisomy 13 or 18
• Monosomy X (Turner syndrome)

PGT-A helps identify these abnormalities so that only chromosomally normal embryos are considered for transfer.

PGT Is a Screening Test

It’s important to know that PGT is a screening tool, not a diagnostic test. This means there is a possibility of false positives or false negatives. In some cases, an embryo may be labeled abnormal (or normal) when it is not.

Additionally, some embryos may show mosaicism, where some cells are chromosomally normal and others are not. The implications of mosaicism are still being studied, but in certain situations, transferring a mosaic embryo may be considered.

Who Might Benefit from PGT?

PGT may be recommended for individuals or couples who:

• Are age 38 or older
• Have experienced recurrent miscarriages
• Have had repeated IVF failures, despite good-quality embryos
• Have unexplained infertility
• Are dealing with severe male factor infertility
• Are known carriers of inherited genetic conditions (e.g., cystic fibrosis, Tay-Sachs disease, myotonic dystrophy)

There are now over 1,000 single-gene disorders that can be screened through PGT.

How Is PGT Performed?

PGT is done through a procedure called an embryo biopsy, typically on day 5, 6, or 7 of embryo development, when the embryo has reached the blastocyst stage. A few cells are gently removed from the trophectoderm layer (which becomes the placenta), and the embryo is then frozen (vitrified).

The cells are sent to specialized genetic labs such as Cooper Genomics for testing. Once results are available, embryos without the targeted abnormalities can be selected for transfer.

What Are the Benefits of PGT?

• For genetic carriers: PGT-M can help avoid passing inherited conditions to future children.
• For patients with recurrent miscarriages: PGT-A helps avoid transferring embryos that are unlikely to implant or result in a healthy pregnancy.
• For patients with advanced maternal age: As the risk of aneuploidy increases with age, PGT-A can improve outcomes.
• For couples with translocations: PGT-SR (structural rearrangement testing) helps identify embryos with unbalanced chromosome rearrangements.
• For patients with repeated IVF failures: PGT-A can help identify chromosomally normal embryos even when embryo quality appears good under the microscope.

Are There Any Risks or Limitations of PGT?

PGT involves handling embryos at the microscopic level, and while complications are rare, they can occur. The biopsy itself poses minimal risk to the embryo, but not all embryos will develop to the stage needed for biopsy.

Because PGT is a screening test, there is a small chance of false positive or false negative results. Some embryos may also be found to be mosaic, meaning they have both normal and abnormal cells. In select cases, transferring a mosaic embryo may still be an option with appropriate counseling.

In rare instances, testing may be inconclusive, and results may not be available for every embryo.