What is PGT?
Preimplantation genetic testing or PGT was formerly called PGS (preimplantation genetic screening) and PGD (preimplantation genetic diagnosis). The terms were changed following a meeting at the World Health Organization by a large multinational group of experts, including Dr. Davis, our medical director at Cayman Fertility Centre. The reason for the change was to standardise the terms across groups of people working in fertility care globally and to make them more accurate. PGS was changed to PGT-a, where “a” stands for aneuploidy, or an abnormal number of genes. PGD is now called PGT-m, for monogenomic (or single gene) disease. There is one additional term, PGT-sr, which stands for “structural rearrangement”. This is used for people who have a genetic condition called a “translocation”.
How is it done?
PGT is performed by removing a small number of “trophectoderm” cells from the embryo, generally at day 5-6 of develpment. These cells are destined to become the placenta and do not become part of the embryo. These cells are removed using a specifically designed laser. A computer allows the embryologist to precisely control the laser energy output, making this method more precise. Laser takes less time and does not expose the embryos to potentially adverse chemicals, so embryos spend less time out of the optimal culture conditions of the incubator. These sampled cells are then frozen and sent to a genetics lab for analysis, which may take 2-4 weeks. After the cells are removed, your embryos are then frozen at Cayman Fertility Centre until the results are reported. This means you will have to return to CFC for your frozen embryo transfer once the test results identify a genetically competent embryo.
What are the risks?
The removal of the embryo cells does not seem to increase the risk of damage to the embryos however there very few studies reporting this data. Sampling the trophectoderm as we currently do seems to be less damaging than older techniques such as “blastomere biopsy”. Long term risks to the health of the baby from PGT are not know yet either, however the added knowledge of the genetics of the embryo has been shown to decrease the odds of miscarriage and improve the chance of implantation, there by resulting in more live births.
Costs are a major concern as IVF can be very expensive and PGT is an additional expense, and it is not a perfect test. The testing can have an error rate up to 2%, meaning some embryos may be deemed abnormal when they have the right number of chromosomes. Additionally, not all genetically competent embryos implant. For some, the testing can be a very helpful tool but for others it may be an added expense without a robust benefit. Talk to Dr. Davis about the pros and cons of PGT for you.