Preimplantation Genetic Testing (PGT)

Preimplantation Genetic Testing (PGT) is an elective test in which IVF embryos are screened for general chromosome number abnormalities (aneuploidy), or for specific genetic disorders, prior to transfer of the embryo into the uterus.

PGT was previously known as Preimplantation Genetic Screening (PGS) and Preimplantation Genetic Diagnosis (PGD). However, several years ago the International Glossary on Infertility and Fertility Care published a revision to the terms previously known as PGS/PGD. The tests themselves remain unchanged, but the new definition further clarifies which test will be performed.

They are:

  • PGT for aneuploidies (PGT-A) – Previously PGS
  • PGT for monogenic/single gene defects (PGT-M) – Previously PGD
  • PGT for chromosomal structural rearrangements (PGT-SR) – Previously PGS for translocation

Preimplantation Genetic Testing for aneuploidies (PGT-A)

PGT-A is a generalized test in which IVF embryos are screened for general chromosome number abnormalities (or aneuploidy.) Abnormalities in chromosome number can lead to failed IVF cycles, miscarriages, and/or birth defects.

PGT-A greatly reduces the possibility of discovering a serious genetic abnormality like trisomy 13, 18, or 21 during pregnancy and subsequently facing the heart-wrenching decision of whether or not to carry the pregnancy to term.

When is PGT-A offered?

PGT-A testing is offered when certain risk factors are present, including but not limited to:

  • When the woman is 35 years of age or older
  • When there is a history of miscarriages
  • When the woman has experienced previously failed fertility treatments

Because chromosomal abnormalities are the leading cause of miscarriage, we commonly recommend PGT-A for couples who have had previous unsuccessful IVF cycles or who have experienced recurrent miscarriages in order to help improve their chances of a healthy pregnancy.

Couples already undergoing PGT-A to screen for chromosomal abnormalities will be able to know the gender chromosome allowing to select the gender of the embryo transferred, if family balancing is a secondary concern.

Preimplantation Genetic Testing for monogenic/single gene defects (PGT-M)

PGT-M is a test used to screen for specific genetic disorders, both monogenic (single gene defects,) in IVF embryos prior to transfer of the embryo into the uterus.

The first PGT-M baby was born in 1992, when the test was referred to as PGD (preimplantation genetic diagnosis). Both parents carried Cystic Fibrosis, a genetic condition where the body produces abnormally sticky mucus that can cause severe respiratory disease. For this family and so many others who would come after them, PGT-M is the difference between passing on a life-threatening disease and giving their children the best chance at a healthy, happy future.

Since 1992, we have developed the ability to use PGT to test for over 600 genetic diseases including Tay-Sachs, Fragile X, Franconi Anemia, and many others.

The ability to diagnose and exclude embryos with verified genetic abnormalities prior to implantation and actual initiation of a pregnancy offers at-risk patients a way to prevent passing on inheritable genetic diseases before implantation.

PGT-M is also beneficial in that it offers alternatives to the clinical risks associated with chorionic villus sampling (CVS) or amniocentesis, two procedures that can only be performed once a pregnancy is in progress to check for genetic diseases.

Above all, it greatly reduces the possibility of discovering a serious genetic abnormality during pregnancy and subsequently facing the heart-wrenching decision of whether or not to carry the pregnancy to term.

When is PGT-M Indicated?

PGT-M testing is indicated when certain risk factors are present, including but not limited to:

  • When there is a familial history of hereditary genetic disorders
  • When chromosomal abnormalities are present in (or carried by) one or both partners

Couples already undergoing PGT-M to screen for chromosomal abnormalities will also benefit from PGT-A which screens embryos for chromosome number, including the gender chromosomes. This is especially useful if family balancing is a secondary concern.

In the case of sex-linked genetic disorders, such as hemophilia or Duchenne Muscular Dystrophy, which generally affect only male children, PGT-M can be also be used for gender selection to choose only female embryos which will not be affected by the disorder.

Only embryos that are genetically normal are transferred into the patient’s uterus.

Preimplantation Genetic Testing for Structural Chromosomal Rearrangement (PGT-SR)

PGT-SR is a test used to identify embryos that have chromosomal rearrangements. Sometimes embryos have the correct number of chromosomes, but the chromosomes themselves have structural abnormalities, including:

  • Translocation: When there is rearrangement of genetic material between two or more chromosomes
  • Inversion: When segments of a chromosome are reversed end-to-end
  • Deletion: When a segment of a chromosome isn’t present
  • Insertion: When a segment from one chromosome is inserted into another chromosome

Those individuals carrying structural chromosomal rearrangements may have an increased risk of infertility, recurrent pregnancy loss, or offspring with chromosomal abnormalities. PGT-SR screens for the embryos that are chromosomally normal (aneuploid), reducing the risk of passing on these structural rearrangements to offspring.

When is PGT-SR Indicated?

PGT-SR testing is indicated when certain risk factors are present, including but not limited to:

  • When there is a known structural chromosomal rearrangement from one or both contributors of the eggs and sperm

The PGT Procedure

The PGT-A, PGT-M, and PGT-SR process begins with egg retrieval and embryo culture as part of IVF. Once the retrieved egg has been fertilized in the laboratory, the embryo is allowed to develop for five-seven days, at which time we biopsy several cells from what is now the trophectoderm, a layer of cells that will eventually become the placenta.

Under a high-power microscope and with the use of sophisticated, hydraulically-operated, ultrafine microscopic glass instruments, the Fertility Institute’s experienced embryology team stabilizes the embryo and makes a small opening in the shell surrounding the embryo.

This state-of-the-art technique, called “assisted hatching,” involves using a special laser to assist the embryo’s natural hatching process, creating a small window through which the cells emerge.

The small sample is then quickly microsurgically removed, without significantly affecting the embryo, and sent off for analysis.

The embryo is then frozen using vitrification, an ultra-rapid and safe freezing technique, and is paused in its development while the Preimplantation Genetic Testing takes place. When the results are received (usually in 7-10 business days), they will be discussed with the patient, and a frozen embryo transfer (FET) is scheduled. During the FET cycle, only a normal embryo is transferred into the woman’s uterus.