Couples undergoing fertility treatment are forced to learn an entirely new language spoken solely in acronyms. IVF (in vitro fertilization). IUI (Intrauterine insemination). PGD (preimplantation genetic diagnosis). PGS (preimplantation genetic screening). It may seem like no one in this community speaks in complete sentences. Though there is an online dictionary to help decipher this language (seriously, check it out), the focus of this post will be to go a bit further than a dictionary definition of two specific topics: PGS and PGD.
PGS vs. PGD
Preimplantation genetic screening (PGS) is the term used when screening an embryo for normal chromosome number or for a gain or loss of chromosomal material. Preimplantation genetic diagnosis (PGD) is the term used when screening an embryo for a single gene disorder. An important note: neither is considered a diagnostic test, despite the name of the latter.
How is PGS/PGD done?
Both PGS and PGD are performed prior to embryo implantation to help identify specific genetic conditions. These methods can only be used on embryos created through IVF. The procedure begins with the normal IVF process –egg retrieval and fertilization in a laboratory. At certain stages of development, a few cells are removed from the embryo. These cells are tested for chromosome anomalies or certain genetic mutations. Testing at each stage has advantages and disadvantages that can be discussed further with your fertility specialist.
What conditions can PGS screen for?
PGS is used to assess the amount of chromosomal material in an embryo. Many factors can give rise to an increased risk for chromosome anomalies in a child, including advanced maternal age, a history of multiple failed IVF cycles, recurrent unexplained miscarriages and a family history of a balanced chromosome rearrangement. These can result in an error in chromosome number (such as Down syndrome, trisomy 18 and trisomy 13) or in a gain or loss of chromosomal material. Each of these, in turn, can lead to miscarriages, stillbirths, intellectual disabilities and/or birth defects.
What conditions can PGD screen for?
PGD is the term used when testing for single gene disorders. The conditions that are screened for are very specific. For example, an embryo cannot be screened for “autism” but can be screened for Fragile X syndrome, which is associated with autism, if the mother is a carrier of a premutation. The conditions that can be assessed are not limited to certain inheritance patterns but rather, depending on the inheritance pattern, one or both partners must have a specific previously identified mutation to look for in the embryo. Mutations are often identified when there is a family history of that condition or through expanded carrier screening, such as CarrierMap.
We hope to have shed some light on these concepts. If you have any questions, we invite you to leave a comment below!