Preimplantation Genetic Testing (PGT)

Preimplantation genetic testing (or PGT) is a type of genetic test healthcare providers can perform on embryos created through in-vitro fertilization (IVF). It allows your fertility specialist to screen embryos for certain genetic or chromosomal conditions before they transfer the embryo into your uterus.

Embryos with these types of conditions have a lower chance of implantation and a higher chance of miscarriage. Results of PGT testing allows couples to select embryos that don’t contain genetic or chromosomal disorders, which then increases their chances of a healthy pregnancy. It can also prevent them from passing on a potentially life-threatening condition to their child.

PGT is a tool fertility specialists can use to help couples pursuing IVF achieve the best chance at a healthy pregnancy and healthy child. It can prevent you from passing down a genetic disease or continuing with the IVF process when it’s likely to be unsuccessful. It can also help couples select the embryo that has the sex they desire.

PGT is different from a prenatal genetic test called a carrier screen. The genetic carrier screening can detect small changes in the genes of the birth parents. If both you and your partner are carriers of the same condition, the embryo you create has a higher chance of having that single-gene condition. People who are carriers of single-gene conditions may choose to have PGT to test their embryos.

PGT is also called:

• PGT or preimplantation genetic diagnosis (PGD)
• PGS or preimplantation genetic screening (PGS)

Couples considering PGT should talk to their healthcare team about the risks and benefits of testing their embryos. A genetic counselor can also help you understand your situation and recommend the type of testing that’s right for you.

What are reasons for getting PGT?

People undergoing IVF may choose preimplantation genetic testing (PGT) to check embryos for genetic and/or chromosomal conditions. This information can help prevent transferring an embryo with a significant medical condition or one that is less likely to implant and more likely lead to miscarriage.

You may decide to have this test if:

• Either you or your partner has a history of inheritable genetic mutations, including conditions like cystic fibrosis
• You have a history of recurrent miscarriage due to chromosomal abnormalities
• You’re of advanced age and at risk of having children with a chromosomal abnormality, like Down syndrome
• You have unexplained infertility or a history of unsuccessful fertility treatment

Types of PGT

Fertility specialists can perform three different types of preimplantation genetic testing:

1. Preimplantation genetic screening for abnormal chromosome number (PGT-A).
2. Preimplantation genetic testing for monogenic (or single gene) disease (PGT-M).
3. Preimplantation genetic testing for chromosomal rearrangements (PGT-SR).

PGT-A

The “A” stands for aneuploid, which is a condition where a cell has excess or missing chromosomes. It’s one of the leading causes of failed implantation and miscarriage. This type of test checks whether the embryo has 46 chromosomes (23 from each parent). An embryo that has more or fewer than 46 chromosomes is less likely to result in a healthy pregnancy. Examples of chromosomal disorders include Down syndrome, Turner syndrome and Patau syndrome.

PGT-M

This test checks the embryo for a specific condition that one (or both) of the birth parents carries. When both parents are carriers of genetic conditions, the chance of them passing it on to their child is higher. This test checks the embryo for such conditions. Examples of single-gene diseases are sickle cell anemia, cystic fibrosis, muscular dystrophy and spinal muscular atrophy.

It also identifies human leukocyte antigen (HLA)-compatible embryos. This is helpful when a sibling of the developing embryo needs a bone marrow transplant.

PGT-SR

Chromosomal rearrangements describe chromosomes that appear structurally different. The main types of rearrangements are inversion and translocation. Both types mean that the genetic material of the embryo contains errors that could lead to serious health problems and repeat miscarriages.

How does PGT work?

Preimplantation testing is only available for couples pursuing IVF treatment. In a typical IVF cycle, your fertility specialist selects embryos to transfer into your uterus based on their appearance under a microscope. This can give them clues about the health of the embryo.

Preimplantation testing takes this a step further and gives them much more data on the genetic and chromosomal makeup of the embryo. This information allows fertility specialists and couples to select embryos that have the greatest chance of successful implantation and development.

Specific steps in preimplantation genetic testing include teh following:

1. A healthcare specialist called an embryologist fertilizes eggs with sperm in a lab dish and allows them to grow into embryos.
2. Once the embryo reaches a certain stage in development (blastocyst stage), an embryologist removes a few cells from the outside layer of the embryo (this is called an embryo biopsy).
3. They send the cells to a lab for genetic testing.
4. The results of the genetic testing allow your embryologist to exclude the affected embryos from the IVF transfer process. They select embryos that come back normal, as these embryos have the greatest chance of implantation.

Are there risks?

No, there aren’t any additional risks to PGT beyond the typical risks of IVF. Testing a few cells from external layers of the developing embryo hasn’t shown any negative impact on the embryo.

PGT adds time and expense to the IVF process. Talk to your fertility specialist or genetic counselor to see if they recommend preimplantation genetic testing.

What type of results do you get with PGT?

The results you get depend on the type of PGT testing you have, but most come back as either normal or abnormal.

• PGT-A test results are normal if the embryo has 46 chromosomes. Test results will be abnormal if the embryo has too many or too few chromosomes. But it can also come back as mosaic, which means it contains both normal and abnormal cells.
• PGT-M test results will also come back as normal or abnormal for each condition your embryologist screens for. If a result is normal, it means the embryo didn’t have that specific condition. If it comes back abnormal, it means the embryo contains the condition. The result might also come back as a carrier embryo.
• PGT-SR test results typically come back as normal or abnormal. A normal result means your embryologist didn’t detect any rearrangements in the chromosomes. An abnormal result means they found one or more rearrangements.

Are the results accurate?

Preimplantation testing can’t guarantee the embryo will be born without any health conditions. As with most tests, there’s a low chance the results aren’t accurate. For this reason, pregnancy care providers recommend having amniocentesis or chorionic villus sampling (CVS) during pregnancy to confirm the results of PGT.

Why is PGT controversial?

Some people believe certain medical interventions are unethical. Health topics like euthanasia, for example, can often be divisive because everyone has their own opinion of what’s morally right or wrong.

Many people believe that “selecting” embryos isn’t ethical because of a practice called eugenics. Eugenics is selecting or choosing desirable genetic or hereditary traits to improve the human species. Historically, eugenics was discriminatory and led to horrific practices. Critics of PGT say it’s a slippery slope and could lead to a way to reject or eliminate populations of people with genetic disabilities or diseases. But advocates believe it’s a way to prevent unnecessary suffering and pain.

Is PGT legal in the United States?

Yes, it’s legal to have preimplantation genetic testing in the U.S. There’s no government regulation of the practice. This means anyone can have PGT for any reason, and it’s entirely at the discretion of the fertility clinic. This raises ethical questions from many groups of people. The Food and Drug Administration (FDA) is working on ways to regulate PGT. Many experts expect there to be laws in place eventually.