Preimplantation Genetic Screening (PGS), also known as aneuploidy screening, is part of the IVF cycle and is performed to detect abnormalities in chromosomal numbers prior to embryo implantation. Errors in the number of chromosomes increase significantly with age, and hence couples after 35 need to undergo this procedure. Having the wrong number of chromosomes in the embryo reduces the chances of successful implantation and even if it implants successfully, there is a higher risk of miscarriage and pregnancy loss. PGS helps older couples select only embryos with the right number of chromosomes and thus increasing the chances of live births. In fact, thanks to PGS, older couples enjoy similar pregnancy rates as younger ones.
Who is recommended to have PGS?
Older women are more likely to have eggs with the wrong number of chromosomes, so usually, it has been recommended to women who are over 37 years of age. Abnormal chromosomes are assumed to be one of the key reasons why older women have problems conceiving and are more liable to have a miscarriage or a baby with Down’s syndrome. And also in women with several miscarriages or failed IVF attempts without explanation.
However, there is limited evidence to show that PGS benefits these groups.
Preimplantation Genetic Testing (PGT) formerly known as Preimplantation Genetic Diagnosis (PGD) is a procedure involving genetic profiling of embryos obtained from IVF cycles to identify the ones with genetic defects than can be passed down to children. Genetic defects can not only cause miscarriages but can also result in the inheritance of a serious genetic condition. PGT is usually used when either or both the couples carry a known genetic condition which they do not want to be passed to their children. In PGT, we specifically look for well-known genetic disorders (around 100 of them) and take action to eliminate their effect on the children. PGT can ensure a healthy baby without any genetic disorders because the embryos are screened for known mutations before implantation.
PGT involves the following steps:
- The woman is given fertility medication to increase egg production
- The eggs are retrieved and then fertilized with sperm from the partner
- The embryos are then allowed to develop for 5-6 days when they reach the blastocyst stage.
- At this stage, some of the cells on the surface of the embryo are retrieved and the embryos are frozen.
- The cells are sent to a laboratory where they are screened for known genetic mutations. In case the genetic disorder is known in advance, the laboratory specifically looks for that particular abnormality. This process may take 7-10 days.
- Once the embryo free from genetic disorders is identified, it is thawed and then implanted into the uterus.
- After about 2 weeks, a pregnancy test is performed to check if the woman has successfully conceived.
PGS & PGD help by reducing the number of IVF cycles necessary, as we are only choosing to implant healthy embryos resulting in very high rates of successful pregnancies. They can also help avoid multiple pregnancies (twins and triplets) which is usually the case with IVF since several embryos are implanted to increase the chances of a successful pregnancy.
However, one must understand that these tests do not guarantee a hundred percent success, as sometimes, a healthy embryo may give negative results and we need to start the cycle to get new embryos. Also, there might be genetic disorders that might manifest later in life, which are not identified at the embryo stage. And if all embryos have a genetic disorder, then we might have to start the cycle again.