Preimplantation Genetic Testing in IVF
Adapted from "Clinical management of mosaic results from preimplantation genetic testing for aneuploidy (PGT-A) of blastocysts: a committee opinion" published in August 2020 issue of Fertility and Sterility.
The goal of Preimplantation Genetic Testing (PGT) is to increase the probability of a successful treatment outcome by decreasing the likelihood of transferring genetically abnormal embryos.
Unfortunately, testing for genetic abnormality in embryos has not been shown to improve clinical outcomes consistently and may be detrimental for some of the believed indications.
Some embryos that tested genetically “abnormal” and would be discarded could have developed into healthy babies. The most common explanation for this phenomenon is the fact that many human embryos will start their existence as a mosaic of two or more chromosomally distinct cell lines, some of which can be genetically abnormal.
If the tested embryonic cells came from an abnormal cell line, such an embryo would be reported as not genetically normal. It has been recognized that human preimplantation embryos have a high rate of mosaicism estimated to be 3% to 20+%.
A mosaic embryo can “self-correct,” by the genetically normal cell line overgrowing and sequestering abnormal cell lines. There have been over 100 documented live births following the transfer of an embryo, which PGT testing had shown to be mosaic.
There is currently no method available to determine which embryos with mosaic results have the best chance of resulting in a successful pregnancy, or which may have the lowest risks of an undesired outcome.
In several studies reporting live births after the transfer of an embryo with mosaic results, the newborns appear to be healthy. Although this is encouraging, no formal evaluations or longitudinal studies have been conducted. Therefore, these data should be interpreted with caution.
In conclusion, if PGT is added to the IVF treatment, an embryo deemed genetically normal, if available, should be preferentially transferred. If no genetically normal embryos are available, patients should be encouraged to proceed with another IVF cycle with or without PGT. Mosaic embryos should be transferred only as a last resort if genetically normal embryos are unavailable.