Preimplantation Genetic Testing
Preimplantation genetic testing refers to the genetic testing of embryos prior to when they are placed in the uterus. Both PGD/PGS involve the same technique for the removal of a cell from an embryo that was created using In vitro fertilization (IVF). This testing is performed in order to determine if certain genetic anomalies are present in the embryo such as genes for diseases (PGD) or abnormal chromosome number known as aneuploidy (PGS).
PGD: Preimplantation Genetic Diagnosis
Preimplantation Genetic Diagnosis, or PGD, is a diagnostic procedure in which embryos are tested for genetic abnormalities. Screening for a genetic illness can be done by performing PGD for a single gene mutation. In this situation, the embryos are screened not for a chromosomal problem, but for a single gene abnormality associated with a single disease. Thousands of gene loci have been identified. Certain illnesses are carried as traits by parents. When both traits are passed on to a child, she or he can be affected. Yet, for many such illnesses, carrier status can be diagnosed prior to conception. Both family history and ethnic background help to determine which testing may be appropriate for you. PGD screenings for single gene defects have been carried out for scores of genetic illnesses, including: cystic fibrosis, Huntington’s chorea, Marfan syndrome, Tay-Sachs and Sickle Cell Anemia.
PGS: Preimplantation Genetic Screening
PGS, Preimplantation Genetic Screening is when embryos are tested for chromosomal abnormalities. Chromosomes are sub-cellular structures that house one’s genes. All of the information that is needed to lead to an individual’s development is contained on these chromosomes and each one of us has 23 pairs of them.
During human reproduction, it is not unusual to produce embryos that have too few or too many chromosomes, this is known as “aneuploidy”. PGS, also commonly referred to as Aneuploidy Screening is used to screen for commonly encountered embryonic chromosomal disorders. Infertile couples may benefit from this test if the female partner is older, if they have had several failed IVF cycles, or if they suffer from repetitive older miscarriages.
The PGD/PGS Process
The embryo is developed for five days in the lab after egg harvesting and fertilization. The outer area of cells, known as the trophoblast (not destined to be the baby) is biopsied. A few cells are removed from over one hundred at this location. The embryo is not harmed or compromised, but occasionally its growth may be arrested. For (PGD) genetic diseases (A-B), the cells are analyzed for markers of these diseases. The embryos are frozen while awaiting results, which can take a few days.
Once the normal embryos that do not carry the genetic disease are identified, the embryos can be subsequently thawed and transferred back without any decrease in success from being frozen. For (PGS) aneuploidy screening (C-E), the cells are analyzed within 24 hours with results of up to all 23 chromosomes and sex chromosomes. This assessment of the integrity of the genetic composition of the embryo allows for a more correct decision as to what embryo to transfer. The embryo is also best frozen and transferred during a subsequent cycle. *Please note that if needed, both PGD and PGS can be done on the same biopsy.