Preimplantation Genetic Testing (PGT) of Embryos

Essentials

  • What is PGT ? (jump to topic): Preimplantation genetic testing (PGT), formerly known as PGS or PGD (preimplantation genetic screening/diagnosis), is a screening technique used to evaluate an embryo’s chromosomal makeup or to determine whether embryos produced through IVF contain genetic abnormalities that may cause birth defects or disease in the child.
  • PGT for chromosomal and genetic abnormalities: Includes three specific types of testing, PGT-A, PGT-M and PGT-SR.
  • Why consider PGT?: Because more than half of miscarriages stem from chromosomal abnormalities, the test is helpful in identifying genetically normal embryos.
  • Who do we recommend PGT for? Women over age 35, parents who are carriers of inherited genetic disorders, or those who have experienced repeated miscarriage or unsuccessful infertility treatments.
  • Risks of PGT: Due to embryo handling and biopsy, a small percentage of embryos may be damaged during PGT and not available for transfer; and as with any test, it is not 100% accurate. Our fertility specialists discuss all of these factors with each patient considering PGT.
  • How genetic abnormalities are passed down: Genes play an extremely important role in human development, so PGT tests the 23 pairs of inherited chromosomes from the mother and father.

What is preimplantation genetic testing (PGT)?

One of the great benefits of in vitro fertilization (IVF) fertility treatment is the opportunity to examine embryos prior to embryo transfer for potential genetic abnormalities that can cause implantation failure, miscarriagebirth defects or lifelong debilitating disease in a child.

Through diagnostic screenings called preimplantation genetic testing (PGT), an embryologist can check embryos at the cellular level for chromosomal abnormalities related to the number of or the rearrangement of chromosomes, which can lead to syndromes such as Down syndrome or to early pregnancy loss. Additionally, the embryologist can check abnormal genetic arrangements that can cause diseases, such as sickle cell anemia or cystic fibrosis.

Fertility specialists recommend PGT, formerly called preimplantation genetic screening (PGS) and preimplantation genetic diagnosis (PGD), for some couples because research shows genetic abnormalities can be a cause of infertility or recurrent pregnancy loss (miscarriage).

 

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PGT for chromosomal abnormalities and genetic abnormalities

There are three types of preimplantation genetic testing, which relate to the previous approaches of embryo diagnosis (PGD) and screening (PGS).

1

PGT-A

PGT-A stands for preimplantation genetic testing for aneuploidy, which is one extra or one too few chromosomes in a normal pair. Aneuploidy is a major genetic cause of embryo implantation failure (meaning no resulting pregnancy), miscarriage or birth defects or disorders. Examples are Down syndrome and Turner syndrome.


2

PGT-M

PGT-M, preimplantation genetic testing for monogenic disease, looks for a specific genetic mutation that one or both parents are at risk of carrying. These include BRCA1 and BRCA2 (connected with breast cancer), sickle cell anemia and muscular dystrophy.


3

PGT-SR

PGT-SR, preimplantation genetic testing for structural rearrangement, evaluates embryos of parents who have a chromosomal structure rearrangement. These structural rearrangements are generally a chromosome inversion (breakage) or a translocation (breakage and reattachment to another chromosome), and result in embryos not likely to develop into a successful pregnancy or birth, resulting in recurrent early pregnancy loss.

 

Why consider preimplantation genetic testing (PGT), formerly PGS and PGD

Ensures healthy embryos are transferred

Genetic testing of embryos is a proven way to increase the chances of IVF success for some patients. One reason is that more than half of miscarriages are caused by abnormal chromosomal conditions. Transferring embryos whose chromosome numbers have been checked may improve the chances for a stable ongoing pregnancy and minimize the potential heartbreak for expectant parents due to an early pregnancy loss.

Helps women in their 40s conceive

It also helps women who are in their upper 30s and lower 40s conceive. The risk of trisomy, a chromosomal condition characterized by one additional chromosome, rises to more than 30% for women in their 40s. Studies show that as a woman ages, eggs that are being matured for retrieval or ovulation are more likely to end up with the wrong number of chromosomes (errors in meiosis). And once fertilized are more likely to divide chaotically (errors in mitosis). The end result is an embryo or conceptus that has abnormal chromosome numbers.

 How Does Age Affect IVF Success?

Genetic testing before pregnancy

PGT before pregnancy reduces the risk that the conceptus will be abnormal. This may reduce the need for invasive testing, such as amniocentesis, once the patient is pregnant. The amniocentesis procedure involves the insertion of a needle into the amniotic sac to test the fluid excreted by the fetus for chromosomal abnormalities. Despite having had PGT, some moms-to-be may still consider having newer, less invasive screening, using cell free fetal DNA found in the maternal bloodstream. This is a blood test that can be done after 10 weeks gestational age and can confirm the PGT results.

Lowers risk of multiples (twins or more)

PGT also minimizes some risks associated with multiple pregnancy (twins or more). The test identifies the healthiest embryos for implantation, often enabling doctors and the family to transfer only one embryo, which greatly reduces the chance of multiples. This is particularly helpful for an older patient in whom transfer with more than one embryo would be considered.

What is the safest number of embryos to transfer?

Reveals gender of embryo

As a result of PGT, intended parents may also know the sex of the embryo, if they so choose.

When is preimplantation genetic testing performed?

An embryologist performs the biopsy to retrieve cells for the PGT when the embryo reaches the blastocyst stage, usually by day 5 or 6 of embryo development. On the day of the biopsy, our expert embryologists removes approximately 5 to 10 cells from the developing blastocyst, and the cells will undergo DNA sequencing and molecular testing at The Fertility Institute’s state-of-the-art lab.

After the biopsy is completed, the embryos undergo cryopreservation and are stored in our laboratory.

Who should consider PGT?

Genetic abnormalities in an embryo may be a cause of infertility and pregnancy loss (miscarriage).

Genetic testing should be considered for patients who:

Risks of embryo PGT

There are a few things to consider when deciding about preimplantation genetic testing.

  • PGT is a screening test, not a diagnostic test. That means it is not 100% accurate and there’s still a small chance of implanting an embryo with a chromosomal abnormality.
  • The biopsy, freezing and thawing of embryos can result in damage to a small percentage of tested embryos, which are then not able to be used for implantation.
  • Some studies suggest that the biopsy may lower implantation ability in some embryos, even if the chromosomes are normal. This effect may be more pronounced in younger patients.
  • Even after having PGT, a pregnant woman may still want to consider getting certain prenatal blood tests to check on the baby’s genetic health.

How genetic abnormalities are passed down

Genes play an extremely important role in human development. They represent the unique DNA sequences that make each person one of a kind.

A human being carries 46 chromosomes organized into pairs – 23 inherited from the mother and 23 inherited from the father – in a strand of cellular DNA, which is present in nearly every cell in the body.

Inherited diseases, traits and disorders, called recessive conditions, are passed from parent to child through mutated or malformed genes that make up the chromosomes. If a person carries only one recessive gene within a pair of chromosomes, he or she will likely not suffer from the disease but has a 50% chance of passing the trait on to a child.

That child will then be a carrier of the mutation but will not be afflicted by the condition. If two parents (or a parent and a donor) each carry the same recessive gene mutation, then their offspring (or embryos) have a 1 in 4 chance of being significantly afflicted with the condition.

Some of the more common genetic issues we test for include spinal muscular atrophy, cystic fibrosis, sickle cell anemia and Jewish genetic diseases, such as Tay-Sachs.

Preimplantation genetic testing helps parents discover whether their embryos have a chromosomal abnormality or a genetic mutation that could impact the likelihood of successful implantation and healthy development. Prior to IVF or fertility treatment, patients can be screened (blood test) to see if they are carriers of a gene mutation. If they are both carriers of the same gene mutation, then IVF with PGT will help prevent the passage of that affliction to a child.