When we talk about genetic testing (pre-implantation) of embryos derived from IVF, we need to distinguish between PGD, or preimplantation genetic diagnosis, and PGS, which stands for preimplantation genetic testing. The two are often confused because they are very similar. In fact, the technology is essentially the same. The difference lies in the type of patient who will use either method.
The differences between PGS and PGD
PGD (Preimplantation Genetic Diagnosis) is used when there is a known risk that parents will pass on a particular abnormality to their offspring. It is a technique that has been established for almost 30 years. In countries where PGD is allowed, couples who would be able to have a child naturally, use IVF to test the embryo(s) created in the laboratory for a specific genetic disease before implantation, avoiding termination of the pregnancy in case of carrying an abnormal embryo.
For example, if both parents are carriers of the thalassemia gene, thanks to PGD, they can only transfer the healthy embryos and not those that are sick.
PGS (Preimplantation Genetic Screening) is a more recent technique, which consists of screening the genetic material of the embryo without targeting a specific abnormality. This involves counting the number of chromosomes, as a healthy embryo should have exactly 46. Trisomy 21, also known as Down syndrome, is one of the abnormalities that PGS can detect. Since the 1990s, PGS has become increasingly common. Women who choose to have a child through IVF often do so because they are getting older or because they have already experienced fertility failures or problems. They seek to maximize their chances of having a child by choosing the “healthiest” chromosomally healthy embryo. Some see this as a hopeless opportunity to improve the chances of successful in vitro fertilization (IVF), especially for women over 35. Others question its effectiveness, believing it stigmatizes disability and, above all, that it tends towards eugenics practices. A PGS test is performed by taking a cell from the embryo, automatically making it less viable although potentially healthy. Therefore, in cases when a woman has few eggs – and therefore embryos – the risk is high. This is precisely why many people’s opinions differ, while prenatal tests can detect any illness and termination of pregnancy in the first trimester.
Why choose preimplantation genetic diagnosis?
PGD can be seen as an extremely early prenatal diagnosis. In case you are at risk of passing on an inherited disease to your child, you can also undergo invasive prenatal testing e.g., chorioparacentesis (chorionic villi from the placenta), amnioparacentesis [amniotic fluid puncture], umbilical cord puncture to determine if the fetus has inherited the condition. These tests are called “invasive” because they involve removing material (tissue or fluid) from the fetus, uterus, or placenta. In the case of PGD, the diagnosis is not made during pregnancy, but in embryos obtained after IVF, before one (or more) of them is re-implanted in the uterus. This gives the woman the chance to become pregnant without the genetic defect being passed on to the unborn baby. This eliminates the possibility of the difficult decision to terminate the pregnancy.
Reducing miscarriages?
Professor René Frydman, “father” of the first French tube baby, and a leader in assisted reproduction, is a strong advocate of extending PGD to other genetic analyses of the fetus, including PGD-A. “Sixty percent of embryos that are reimplanted will never develop,” he says. “So, we could know in advance which ones can be implanted, but we choose not to use it as we should. Meanwhile, women continue to have high rates of miscarriage…”
Success, reliability and safety
PGD is a highly reliable diagnostic technique that comes with a minimal risk of misdiagnosis (<1%). During PGD, we carefully remove one or even two or more cells from an oocyte or multicellular embryo. As a rule, the embryo remains intact and develops normally after this procedure. In assisted reproduction, it is important that all necessary tests are carried out without overdiagnosis and overtreatment. Before deciding whether PGD will be helpful for you, it is important to analyze each case individually to evaluate the advantages and disadvantages.
