Preimplantation Genetic Screening / Diagnosis (PGS / PGD)

Preimplantation Genetic Screening (PGS)

PGS, also known as aneuploidy screening, is a method for determining if an embryo has the proper number of chromosomes. Tests are performed on sample tissue taken from the embryo in question. Although PGS does not test for a particular inherited disease, aneuploidy (chromosomal number abnormality) can occur quite apart from any genetic predisposition, and this method may still be employed by couples with no known genetic disorders.

Preimplantation Genetic Diagnosis (PGD)

PGD is a method of genetic screening used to determine if an embryo carries the genes for a particular disease or chromosomal anomaly, such as breast cancer, sickle cell anaemia, or chromosome translocations. For this purpose, a DNA probe—a means of locating an individual gene in a DNA sequence—is used to test for predisposition to the disease or anomaly in concern, within a tissue sample taken from the embryo. This probe is designed based upon the unique results of the couple’s genetic analyses. The information thus obtained, for some couples, outweighs the risks associated with this method (see below) and affords embryologists a level of diagnostic specificity even greater than with PGS.

Both PGD and PGS do have some drawbacks. The biopsy and freezing processes involved can harm the delicate embryos. Even under the best conditions, each test is invasive, and studies have shown that the probability of conception diminishes after either is performed. Likewise, the minuscule tissue sample used for testing provides only partial data and may yield false test results. This can lead to transfer of an abnormal embryo and subsequent miscarriage or to the discarding of healthy embryos.

Then there is the question of how unused embryos are handled. Usually, healthy embryos are frozen for later use or donated to infertile couples. But those embryos deemed unfit for use are frequently discarded or donated for scientific research, which may raise moral concerns for couples who consider all embryos to be human life. Lastly, although they can reveal much about the genetic makeup of an embryo, neither test is capable of screening all types of genetic disorders or defects. Despite these drawbacks, PGD and PGS have been shown to significantly reduce the chances of miscarriage and genetic defects in most cases.