Genetic Testing of Embryos: Preimplantation Genetic Testing (PGT)
What is PGT?
Preimplantation Genetic Testing (PGT) is a state-of-the-art reproductive technology used in conjunction with in vitro fertilization (IVF) to identify embryos free of chromosomal abnormalities and specific genetic disorders for transfer to the uterus. These genetic conditions can interfere with embryo implantation, result in pregnancy loss, or result in the birth of a child with physical or cognitive abnormalities. By identifying embryos that have tested negative for certain genetic abnormalities, we can improve the chances of a pregnancy occurring, as well as increase the probability that those pregnancies that do occur will be healthy and unaffected by the specific genetic condition(s) tested for. PGT can only be performed within the context of an IVF cycle where eggs are fertilized with sperm in a laboratory and develop into embryos.
There are two main types of PGT: Preimplantation genetic screening (PGS), which tests for chromosome abnormalities such as Down syndrome, and Preimplantation genetic diagnosis (PGD), which tests for specific single gene disorders. You may have also heard this technology referred to as Comparative genomic hybridization (CGH), which is the specific technique most commonly used to test for chromosome alterations.. It is important to note that embryo testing via PGT does not test for all genetic disorders, only those caused by chromosome abnormalities or a specific single gene disorder that the embryo is at increased risk for based on medical or family history.
Who Should Consider PGT?
PGT may be beneficial for many couples. If you say "yes" to any of the following questions, PGT may be something to consider and discuss with your RBA physician:
- Are you a woman over 35?
- Have you and your partner had multiple miscarriages or failed IVF cycles?
- Do you or your partner carry a chromosome rearrangement, such as an inversion or translocation?
- Are you or your partner carriers of a specific genetic disorder (i.e. cystic fibrosis, sickle cell disease, fragile X syndrome, or other disorder)?
- Do you have a long history of unexplained infertility?
How is PGT performed?
With PGT, the egg is fertilized in a laboratory via standard IVF procedures. Then, in what is known as an embryo biopsy, cells are removed from a blastocyst stage embryo on day-5 or day-6 of development. While biopsy can be performed at earlier embryo stage, such as day-3, it is typically avoided as it has more potential risk to the embryo. The cells are then examined for genetic abnormalities using one of several techniques known as Comparative Genomic Hybridization (CGH), Fluorescent in Situ Hybridization (FISH), direct mutation testing, and linkage analysis. The particular PGT technique used is dependent upon each patient's specific circumstances, as well on which genetic laboratory is performing the testing, and will be discussed with your RBA physician.
Is PGT Safe? What are the risks to the resulting child?
Yes. Data from many years of PGT and thousands of live births have shown no reported increase in congenital abnormalities over that seen in the general population. Despite these encouraging results, it is important to emphasize that PGT is a still a relatively-new procedure, and long-term follow-up of these children is needed to assure the procedure's overall safety.
How Accurate is PGT?
Clinical misdiagnosis, in which a genetically-abnormal embryo thought to be normal is transferred or a genetically-normal embryo thought to be abnormal is discarded, is a risk. Depending on the particular situation, misdiagnoses are believed to occur in about 5-10% of embryos. There is also a small risk that a cell will be non-analyzable or only partially-analyzable. Embryos that have no PGT results may be transferred if they demonstrate normal development, but the benefits of PGT will not apply. In addition, PGT cannot detect abnormalities of chromosomes or genes which are not specifically included in the test, nor can it detect abnormalities if they occur in only some of the cells of the embryo (mosaicism). It is highly recommended that traditional prenatal diagnosis in the form of chorionic villus sampling (CVS) or amniocentesis is considered if a pregnancy is established after PGT.
Risks of PGT
PGT is a highly technical procedure, requiring a high level or expertise by laboratory personnel performing each step of the procedure. At RBA, embryologists performing the biopsy and fixation procedures have extensive experience in micromanipulation and cytogenetic techniques, and the embryo damage rate is minimal (<1%). In the rare cases that an embryo is damaged, it can be identified right away by viewing it under a microscope, and would not be selected for transfer. There is some evidence that embryo biopsy may slightly reduce the chance for pregnancy implantation. The amount of reduction is unclear and RBA has had over a thousand pregnancies resulted from embryos in which PGT was used.
What happens if there are healthy embryos that are not used?
All decisions about which embryos to transfer and how to use excess remaining embryos are made together with the couple and their RBA physician.
Feel free to contact our genetic counselor, Jamie Dokson, for any additional questions regarding PGT.