What Is Preimplantation Genetic Diagnosis (PGD)?

Genetic Testing with IVF Has Benefits and Controversies

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Preimplantation genetic diagnosis (PGD) is a procedure used with in-vitro fertilization (IVF) to screen for a specific genetic abnormality before transferring the fertilized eggs into the mother.

PGD is a type of preimplantation genetic testing. The other type of preimplantation genetic testing is called preimplantation genetic screening, sometimes referred to as PGS, or PGD for aneuploidy screening.

What Is Preimplantation Genetic Testing?

PGD is a genetic test performed on embryos before they are implanted in the mother during a cycle of IVF. During this procedure, the technician removes one or two cells from each embryo about three days after fertilization. These cells undergo genetic testing to look for genetic defects.

As its name suggests, preimplantation genetic testing is done before the embryo is implanted in the mother and before she becomes pregnant. It therefore eliminates the difficult decision of continuing or terminating a pregnancy if an abnormality is discovered during pregnancy by the prenatal diagnostic tests amniocentesis or chorionic villus sampling.

Another important goal of preimplantation genetic testing is to improve IVF success by choosing embryos with no genetic abnormalities.

Who Gets the Test?

Preimplantation genetic testing is only used in couples undergoing IVF.

According to the Society for Assisted Reproduction Technology, 5-percent of IVF cycles in 2012 used preimplantation genetic testing. The two types of preimplantation genetic testing are:

  • Preimplantation genetic diagnosis (PGD). In this case, one or more parent has a genetic disease or a mutation that carries a risk of disease, such as cystic fibrosis, Tay-Sachs disease, sickle cell disease, Huntington’s disease, hemophilia or a BRCA gene mutation. Or they have chromosome translocations, which can cause implantation failures, recurrent pregnancy losses or mental or physical problems in the baby. The test diagnoses or rules out the same disease, mutation or chromosome translocation in the embryos.
  • Preimplantation genetic screening (PGS). This test looks for an abnormality called aneuploidy - an extra copy (trisomy) or missing copy (monosomy) of a chromosome - in the embryos. PGS is often used when a genetically normal couple has recurrent unexplained miscarriages or recurrent miscarriages with known aneuploidies.

Why Preimplantation Genetic Testing Is Controversial

Some uses of preimplantation genetic testing are controversial. While experts generally agree that it's appropriate to use PGD to diagnose genetic defects when the parents are carriers, the use of PGS to look for aneuploidies in genetically normal parents has been criticized.

Aneuploidy is the leading cause of early pregnancy loss. Some practitioners recommend the technique for genetically normal couples experiencing recurrent miscarriages with the idea that pre-screening might reduce the incidence of miscarriages due to aneuploidies.

Some clinics also offer PGS in cases of:

However, the evidence is not yet conclusive on whether or not preimplantation genetic screening is helpful for increasing successful pregnancy rates. In fact, the Society for Assisted Reproductive Technology (SART) and American Society for Reproductive Medicine (ASRM) say that the evidence that's currently available does not support the use of PGS to improve the improve the rate of live births in women with advanced maternal age, recurrent pregnancy losses or implantation failures.

Other possible uses of preimplantation genetic testing are also controversial, such as:

  • universal screening for a neuploidies
  • "family balancing" (choosing the sex of your child)
  • conceiving a child who will be a matching tissue donor for an ill sibling
  • choosing physical characteristics of a child, such as hair color

Deciding to Get PGD or PGS

If you are undergoing IVF, you will probably be offered preimplantation genetic testing. It's up to you and your partner to decide if preimplantation genetic diagnosis and/or preimplantation genetic screening are tests you want to undertake. Before you agree to them, meet with a genetic counselor so you fully understand the possible outcomes of the results.

Sources

Preimplantation Genetic Diagnosis. Medscape News and Perspective. December 30, 2015.

Brezina, P.R., Kutteh, W.H. (2015). Clinical applications of preimplantation genetic testing. BMJ.

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