Preimplantation Genetic Screening (PGS)

Definition - What does Preimplantation Genetic Screening (PGS) mean?

Preimplantation genetic screening (PGS) is a procedure in which the genes of embryos created through in vitro fertilization (IVF) are examined for a number of potential genetic disorders before being transferred into a uterus.

Embryos with chromosomal abnormalities are unlikely to successfully implant and often result in miscarriage. PGS is sometimes offered to people with recurrent pregnancy loss or infertility to increase their chances of a successful pregnancy.

FertilitySmarts explains Preimplantation Genetic Screening (PGS)

Preimplantation genetic screening (PGS) is used to screen for embryos with an abnormal number of chromosomes. An abnormality in the number of chromosomes an embryo contains can result in miscarriage, Down's syndrome, or Edward's syndrome. Embryos found to have a genetic abnormality are not typically used for transfer with the goal of increasing the chance of a successful pregnancy and a healthy child.

Although similar, PGS differs from another available preimplantation test, called preimplantation genetic diagnosis (PGD). PGS is used to identify embryos with unusual numbers of chromosomes, whereas PGD is used to identify embryos that carry a single-gene disorder in cases where the risk of a specific gene mutation is higher.

In other words, PGS is used to screen out chromosomally abnormal embryos and PGD is used to diagnose specific defects that an embryo is likely to inherit. PGD is offered to individuals with a family history of serious genetic disorders, such as Cystic Fibrosis, or who have another biological child with a genetic disorder. If indicated, PGS and PGD may be performed at the same time.

PGS involves collecting and karyotyping a sample of cells from the developing embryo. The type of procedure used to obtain cells for genetic testing is dependent on the stage the embryo is at in its development. Embryos can be biopsied on day 3 or 5 to obtain cells for testing.

Typically, health care providers prefer to wait until day 5, when an embryo is at the blastocyst stage of development. This is because a blastocyst contains approximately 200 cells, so removing a few are thought to be less destructive to the developing embryo than at earlier stages of development. Cells are removed from the blastocyst during a trophectoderm biopsy and screened for genetic abnormalities prior to implantation.

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