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Preimplantation Genetic Testing (PGT)


Preimplantation Genetic Testing or PGT (Previously called Preimplantation Genetic Diagnosis or PGD) is a method that enables testing genetic disorders in embryos before embryo transfer. To achieve this, one to few cells are removed from the embryos on either third or fifth day of development via a microscopic procedure in the embryology lab. These cells then are subjected to genetic analysis, normal embryos are selected and transferred by day 5 or 6 of the development. Common indications of PGT include testing chromosome numbers (aneuploidy screening: PGT-A), testing for single gene (mono-genic) disorders (PGT-M) (e.g., sickle cell disease, Fanconi’s anemia, cystic fibrosis, BRCA mutations, Retinoblastoma gene, others, conceiving an HLA-matched sibling for bone marrow transplantation, chromosome translocations (PGT-SR), gender preference (due to sex-linked disorders or family balancing).

PGT-A is the most common type of genetic testing being performed on embryos. Around 4-5 cells are removed from an embryo for the genetic testing and sent to the genetic testing laboratory for analysis. Typically, it takes around a week to receive the results of genetic testing. For this reason, biopsied embryos are frozen until the test results are received. Based on those results, a single embryo with normal complement of chromosomes (euploid) is selected for a frozen embryo transfer on the patients succeeding cycle. The advantages of transferring a genetically normal embryo include higher probability of pregnancy compared to those untested, reduced miscarriage rates, lower chance of multiple gestations, and the option to select a certain gender for family balancing. Some of the disadvantages are the additional costs of embryo biopsy, genetic testing, and the subsequent frozen embryo transfer cycle. In addition, there is the possibility of receiving inconclusive results especially due to mosaicism in embryos. This means that some cells may have normal chromosomes, but others may not in the same embryo. Many of these embryos may not implant, miscarry, or result in complicated pregnancies. Some others may still do well, but we currently do not have a reliable method to differentiate a “good” mosaic embryo from the “bad” one.

We have diagnosed many genetic conditions with PGT, but new diseases are continuously added to the list. As long as the genetic mutation or genes related in transmission are known, a diagnostic strategy can be developed. However, for rare disorders, molecular probe preparation can take weeks to months and hence advance planning is important.

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