For pregnancy to occur, the embryo should not be only beautiful, but also euploid, that is, contain an equal number of chromosomes (a total of 23 pairs) from mom and dad. Embryos with chromosomal abnormalities either do not implant, or lead to missed abortion (about a half of missed abortions is due to genetic causes), and in certain situations – to birth of a child with certain developmental abnormalities (the most common example is Down syndrome, where an additional 21st chromosome is present in the genotype).
Unfortunately, embryologists are not able to see individual chromosomes in a microscope and to judge the genetic status of an embryo by its appearance: embryos with excellent morphology can carry various aneuploidies, and vice versa, there are blastocysts with a poor morphological assessment, but with a normal set of chromosomes. Preimplantation genetic testing for aneuploidy (PGT-A) helps us to learn about the chromosome set of the embryo, so we can choose only euploid blastocysts for transfer.
The frequency of aneuploidy occurrence is interrelated with age and can account for up to 86% of abnormal embryos in women over 40 years old. However, aneuploid embryos are found not only in patients of older reproductive age. For example, proportion of aneuploid embryos in patients under 35 years may be up to 50% of their total number, therefore, the negative result of embryo transfer without PGT-A may be due to embryo aneuploidy. The same is true for programs with donor oocytes.
The gold standard for PGT is trophectoderm biopsy – sampling of cells at the blastocyst stage (5-6 days of embryo culture), which allows analyzing only those embryos that are more likely to achieve pregnancy. Trophectoderm biopsy does not affect those cells from which the embryo itself will be formed later (ICM), and a well-organized biopsy procedure does not reduce the likelihood of embryo implantation, so the procedure is totally safe.
Conducting an aneuploidy assay (PGT-A), we are able to see only numerical changes in the chromosome number, as well as significant changes in their structure, but we cannot guarantee the absolute health of the unborn child. In some cases, if there are indications according to the geneticist, it is necessary to conduct an analysis for monogenic diseases (PGT-M), or for structural rearrangements (PGT-SR); these analyses are necessary only for a small proportion of patients and require a long preliminary preparation.
PGT-A does not improve the quality of each specific embryo, but it allows us not to use those embryos that are meaningless to transfer, and perhaps even dangerous. Thus, the time to the goal achievement is reduced and the proportion of unsuccessful transfers is also reduced. In addition, PGT-A is also valuable in the context of long–term storage of embryos – after all, it is much more logical to have information about the potential of those embryos that will be expectative in a few years.