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Embryo Testing: The Difference Between PGS and PGD

embryo testing

Success rates for IVF have improved significantly with the advent of PGS (preimplantation genetic screening) of embryos. In order to provide genetic results of an embryo, a biopsy of a blastocyst embryo must be removed by a trained embryologist. Today, this procedure is frequently performed as part of in vitro fertilization. Depending on a patient’s reproductive history and personal medical history, a reproductive endocrinologist may offer preimplantation genetic screening and/or preimplantation genetic diagnosis (PGD). In this article, we explain the difference between these two types of embryo testing.

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What is Preimplantation Genetic Screening (PGS)?

Preimplantation genetic screening or PGS is now frequently referred to as preimplantation genetic testing for aneuploidy or PGT-A. This test screens embryos for chromosomal abnormalities. A chromosomally normal embryo will have 46 chromosomes. Any deviation from this number is likely to result in failed implantation, miscarriage, or a viable chromosome syndrome like Down syndrome. 

Although this might sound alarming, chromosomal abnormalities are actually commonplace. Over a third of all embryos from a 30-year-old woman are detected to have a chromosomal abnormality. As women get older, this proportion rises to approximately 80% abnormal by the time a woman is 40. For this reason, women who try to get pregnant later in life may experience difficulties conceiving, an increased chance for miscarriage, and an increased chance of having a child with a chromosomal syndrome like Down syndrome, compared to a younger woman. As a result, reproductive endocrinologists will often recommend PGS to women in their late thirties or early forties. 

What is Preimplantation Genetic Diagnosis (PGD)?

PGD is commonly known as PGT-M or preimplantation genetic testing for monogenic diseases. If a couple has a known genetic disease in their family which is caused by a specific single gene mutation, they can consider preimplantation genetic diagnosis or PGD to test each embryo and determine whether each embryo has inherited the mutation and disease predisposition. This embryo testing can also be utilized when both the egg source and sperm source are carriers of the same autosomal recessive disease and wish to identify whether their embryos have inherited both mutations.

How is Embryo Testing Performed?

Both PGS and PGD require a biopsy of cells from a blastocyst embryo. This procedure is called ‘embryo biopsy’ and is very commonplace in embryology labs across the United States. The vast majority of embryos survive the biopsy although there is a small chance that an embryo could arrest after the biopsy is performed.  There are no studies to suggest an increased risk of birth defects in embryos that are biopsied compared to those that aren’t. 

The biopsied cells are shipped to a lab for testing while the remaining blastocyst embryo is frozen and stored until the results are returned. From here, the patient can make a plan for how to use the frozen embryos. Once results are reported back to the patient, they will work with their doctor to select an embryo for a future transfer. Most doctors will recommend that the couple choose an embryo free from chromosomal abnormalities. If PGD was utilized to test for a specific single gene disease, the patient will typically choose to transfer an embryo that is unaffected with the specific disease. 

When Should You Consider PGS or PGD?

Couples can benefit from embryo testing for a variety of reasons. For example, if a woman is experiencing recurrent miscarriage either naturally or as part of IVF, it may be useful for doctors to screen her embryos for chromosomal abnormalities. Furthermore, there is increasing evidence that PGS is useful for women over the age of 35 undergoing IVF treatment. As discussed above, PGD is useful for couples who have a known genetic mutation in their family. Remember, if you have any questions or concerns about embryo testing, consult with your fertility specialist. They will be able to advise you as to which test is appropriate given your fertility history and specific circumstances.

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