Serena Talcott Baughman, MS, CGC
By Serena Talcott Baughman, MS, CGC; Genetic Counselor
During our genetic consults, patients commonly ask, “What is the difference between PGS and PGD?” Fertility treatment is full of three-letter acronyms and genetic testing is no different. PGS stands for preimplantation genetic screening and PGD stands for preimplantation genetic diagnosis. Let’s explore what these terms mean, what they have in common, and how they differ.
What is Preimplantation Genetic Screening (PGS)?
At Oregon Reproductive Medicine, PGS is also called CCS, which stands for comprehensive chromosome screening. PGS is available to any of our patients undergoing in vitro fertilization (IVF). A few cells are removed from the embryo and sent to the genetics lab where the number of chromosomes is analyzed. A chromosome is the package of genes, or DNA, inside each cell. A normal embryo will have 46 chromosomes and an abnormal embryo will have an extra or missing chromosome. Normal chromosomes are very important for normal growth and development.
The main motivation for this testing is to find the embryo that is most likely to be successful since embryos with the normal number of chromosomes are most likely to implant and become an ongoing pregnancy. If an embryo with an abnormal number of chromosomes is used instead, it will most likely not implant at all or it will result in a miscarriage later. In some cases, embryos with chromosome changes do result in babies, but they would have a condition like Down syndrome, Turner syndrome or trisomy 13. PGS is also able to detect these types of abnormalities.
The chance for chromosome abnormalities increases as women become older. We expect to see about 50% abnormal embryos in a woman who is 38 years old. However, as more patients opt for PGS testing at ORM, we have learned that even with egg donors – fertile women in their 20s – the average rate of abnormal embryos is about 35%.
Initially, most of our patients who opted for PGS were women over 35 years old. Now, >75% of our patients opt to have this screening. Some examples of patients who have done PGS with us over the last month:
- A couple in their 30s who have had three miscarriages. They opted for PGS in order to decrease the chance that they will have another miscarriage after an embryo transfer.
- A couple in their 40s who wanted to be sure they were able to identify a chromosomally normal embryo before considering a transfer.
- A single woman proceeding with IVF using donor sperm. She opted for PGS because she knew it would be difficult to raise a child on her own and wanted to reduce the chance for a child to be born with a chromosomal syndrome.
- A same-sex male couple who will use a surrogate to carry their pregnancy. They stated that they have invested significant financial resources, time and emotion into having a family and it was important to them that they utilize all available testing to maximize the chance that their surrogate becomes pregnant on the first transfer.
What is Preimplantation Genetic Diagnosis (PGD)?
PGD is available to anyone who knows they are at risk of passing on a genetic disease or chromosome abnormality to their children. For example, if one parent has been diagnosed with a genetic disease or syndrome and there is a 50% chance for them to pass this on to their children, they may consider PGD. Another example is that in which both parents are healthy but are carriers of a genetic disease; in this case their children have a 25% risk of being affected with that disease, and they too may consider PGD. (All ORM patients are offered genetic carrier screening to see if they have a high risk of having a child with a genetic disease). This is known as “single-gene PGD“.
In other cases, a healthy parent is found to have a chromosome translocation, which increases the chance that their embryos will have abnormal chromosomes resulting in failed transfers or miscarriages. Chromosomal PGD can be used to identify the embryos that are most likely to develop into healthy babies.
Some examples of patients who have done PGD through our clinic:
- A couple in their 20s who have a daughter with a genetic disease. They are hoping to have another child who is not affected by the same severe condition.
- A woman who was found to be a carrier of a chromosome translocation after four miscarriages.
- A man affected with a heart condition who asked us to facilitate genetic testing. Once his gene mutation was found, he opted to proceed with PGD because of the 50% chance that he could pass this gene mutation to his children.
- A couple who did carrier screening through our clinic and were both found to be carriers of cystic fibrosis. They want to use PGD in hopes of identifying embryos that will not have the disease.
Unlike PGS, PGD is not an option (and is not necessary) for all patients going through IVF. PGD is available to couples who know they have or carry a genetic disease and know their gene mutation. At times, our new patients meet with one of our physicians and they only have a clinical diagnosis of their disease, but know that it may be genetic. Our genetics team can then facilitate appropriate genetic testing and if a gene mutation is found, then PGD can be considered. PGD patients also benefit from PGS – once the patient is found to have embryos that did not inherit the genetic disease, PGS identifies those that are chromosomally normal and most likely to implant.
Figure 1. Overlap between preimplantation genetic screening (PGS) and preimplantation genetic diagnosis (PGD). Preimplantation refers to the embryo being biopsied and tested before it is transferred to the uterus. This applies to both PGS and PGD. All patients who undergo PGD will also have PGS for their embryos. Only a small subset of PGS patients also have PGD.
Whether you are considering PGS or PGD, it is important to understand the potential benefits, risks, and limitations of these tests. One of our genetic counselors would be happy to meet with you to discuss these options.
Please contact us if you would like to discuss these options further.