Understanding Preimplantation Genetic Testing (PGT)

Couples undergoing fertility treatment should make sure they have had appropriate genetic screening. When screening reveals that a couple is at risk of transmitting a serious genetic condition to their children, advances in reproductive medicine may offer a way to significantly reduce that risk before a pregnancy even begins.

Preimplantation Genetic Testing (PGT) allows embryos created through in vitro fertilization (IVF) to be tested for certain genetic or chromosomal abnormalities before an embryo is transferred to the uterus.

For families at risk of passing on a serious inherited disease, PGT can be an extraordinarily valuable tool. In other circumstances, PGT may be considered because of a chromosomal rearrangement, recurrent pregnancy loss, or concerns about the chromosome number of embryos.

Understanding what PGT can and cannot accomplish can help couples make thoughtful and informed decisions together with their medical team.

What Is PGT?

PGT is a laboratory technique used to test embryos created through IVF before they are transferred to the uterus. After eggs are retrieved and fertilized, the resulting embryos are cultured in the laboratory for several days. A small number of cells are then carefully removed from an embryo and sent to a genetics laboratory for analysis.

Depending on the reason for testing, the laboratory may be looking for a particular genetic disease, a chromosomal rearrangement, or an abnormal number of chromosomes.

Because PGT is performed on embryos outside the body, it requires IVF. This means that even couples who have no difficulty conceiving naturally may need to undergo an IVF cycle specifically in order to perform PGT.

The Different Types of PGT

PGT is not a single test. There are three major types, each designed for a different purpose.

PGT-M: Testing for a Single-Gene Disorder

PGT-M (Preimplantation Genetic Testing for Monogenic Disorders) is used when there is a known risk of passing on a specific single-gene condition.

For example, when both partners are carriers of the same recessive genetic disease, each pregnancy generally carries a one-in-four chance that the child will be affected. PGT-M can be used to identify embryos that are unaffected by the particular condition so that an unaffected embryo can be selected for transfer.

Examples may include Tay-Sachs disease, cystic fibrosis, Gaucher disease, and many other inherited conditions.

PGT-M may also be appropriate when one parent carries certain dominant or X-linked genetic conditions.

PGT-SR: Structural Chromosomal Rearrangements

PGT-SR (Preimplantation Genetic Testing for Structural Rearrangements) is used when one parent carries a chromosomal rearrangement, such as a balanced translocation.

A person with a balanced translocation may be completely healthy but can produce embryos containing too much or too little genetic material. This can result in infertility, miscarriage, or an affected child.

PGT-SR can help identify embryos with an appropriate chromosomal arrangement for transfer.

PGT-A: Screening Chromosome Number

PGT-A (Preimplantation Genetic Testing for Aneuploidy) screens embryos for abnormalities in chromosome number.

Embryos normally contain a specific number of chromosomes. When an embryo has an extra or missing chromosome, this is known as aneuploidy. Aneuploid embryos are a common cause of failed implantation and miscarriage, and the likelihood of aneuploidy increases with maternal age.

PGT-A may therefore be considered in certain IVF cases, including advanced maternal age, recurrent pregnancy loss, or repeated unsuccessful IVF treatment.

However, PGT-A is different from testing for a known inherited disease. Its role and benefit in IVF treatment are more nuanced and continue to be debated among fertility specialists. It is not necessary or appropriate for every patient undergoing IVF.

Who Might Consider PGT?

PGT may be considered when:

Genetic carrier screening before marriage or pregnancy can play an important role in identifying genetic risks early. When screening reveals a shared genetic risk, couples can meet with a genetic counselor and fertility specialist to understand whether PGT is an appropriate option.

How Does PGT Work?

PGT requires a full IVF cycle. The process generally includes:

Ovarian stimulation: The woman receives medications that encourage multiple eggs to mature during a single cycle.

Egg retrieval and fertilization: The eggs are retrieved and fertilized in the laboratory.

Embryo culture: The resulting embryos are cultured for several days, generally until the blastocyst stage around day five or six.

Embryo biopsy: A small number of cells are carefully removed from embryos that have developed sufficiently for biopsy. Not every embryo will necessarily develop to the stage at which biopsy can be performed.

Genetic analysis: The biopsied cells are sent to a specialized genetics laboratory and analyzed for the specific condition or chromosomal issue being investigated.

Embryo selection and transfer: Based on the results, an appropriate embryo may be selected for transfer to the uterus. Other suitable embryos may be frozen for possible future use.

Because genetic testing generally takes time, embryos are commonly frozen after biopsy and transferred during a later cycle once the results are available.

What Can PGT Accomplish?

For couples at significant risk of transmitting a known genetic disease, PGT can dramatically reduce the likelihood of having an affected child.

It can also allow couples to know which embryos appear unaffected by a particular condition before pregnancy rather than discovering the condition through prenatal testing later in pregnancy.

For families who have experienced the effects of a serious genetic illness, this can provide an important path toward building a healthy family.

What Are the Limitations?

PGT is a powerful technology, but it does not guarantee a healthy baby.

Testing generally evaluates embryos for the particular genetic or chromosomal issue being investigated. It cannot rule out every possible genetic disorder, birth defect, developmental condition, or pregnancy complication.

Results can occasionally be inconclusive, and not every embryo will develop sufficiently to be biopsied or transferred. In some IVF cycles, testing may reveal that there are no suitable unaffected embryos available for transfer.

Embryo biopsy also carries a small potential risk to the embryo.

Couples considering PGT should therefore have realistic expectations and discuss the accuracy and limitations of their particular test with their fertility specialist and genetic counselor.

When Should You Speak With a Professional?

Consider speaking with a genetic counselor and fertility specialist if you and your partner have carrier screening results showing a shared genetic risk, if a serious genetic condition runs in either family, if a parent carries a chromosomal rearrangement, or if you have had a previous pregnancy or child affected by a genetic condition.

Couples experiencing recurrent pregnancy loss or repeated IVF failure may also want to discuss whether chromosomal testing has a role in their particular situation.

PUAH's team can help couples understand the medical options, connect with appropriate specialists, and navigate the different stages of fertility treatment.

Summary

Preimplantation Genetic Testing allows embryos created through IVF to be examined for certain genetic and chromosomal abnormalities before transfer.

For couples at risk of transmitting a serious inherited disease, PGT-M can help identify unaffected embryos. PGT-SR may be used when a parent carries a structural chromosomal rearrangement, while PGT-A screens embryos for abnormalities in chromosome number and has a different and more nuanced role in fertility treatment.

PGT can be an extraordinarily valuable tool, but it also has important limitations. With appropriate genetic counseling and fertility care, couples can determine whether PGT is appropriate for their particular circumstances.

PUAH is available to help families understand their options and navigate the process with clarity and confidence.

Disclaimer: This article is intended for educational purposes only and should not replace medical advice from a qualified healthcare professional.