A couple of weeks ago, I had shared with you about one of our Success Stories regarding a patient who after multiple tries at conceiving had undergone Pre-Implantation Genetic Diagnosis at Global Fertility Solution. In that blog I had extensively spoken about PGD and what it entails. After the blog I received a bunch of questions asking about PGS and whether that is the same as PGD; and if not how was it different and which one is more helpful. So today let me shed more light on these two terms and the difference between the two when it comes to IVF.
Pre-implantation Genetic Diagnosis (PGD)
Pre-implantation genetic diagnosis (PGD) involves screening single cells from embryos for genetic diseases and chromosomal disorders. This testing is performed with in-vitro fertilization (IVF) and occurs prior to embryo transfer, before a pregnancy is established. 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.
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.
PGD offers at-risk couples the opportunity to select embryos for transfer based on their genetic and chromosomal status. The goal of PGD is to identify abnormal embryos, so they will not be transferred, leaving unaffected embryos to be selected for transfer that are more likely to make healthy, disease-free babies.
PGD can identify the presence of chromosomal translocations (rearrangements of parts of chromosomes) and single-gene disorders (genetic diseases that are the result of a single mutated gene). There are over 4,000 single-gene disorders. The most common diseases tested for are Cystic Fibrosis, Tay Sachs, Fragile X, Myotonic Dystrophy and Thalassemia.
Pre- implantation Genetic Screening (PGS)
PGS is available to all 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 are analysed. 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, 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, more than 75% of our patients opt to have this screening.
The difference between PGD & PGS:
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, new patients only have a clinical diagnosis of their disease, but know that it may be genetic. The 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.