The alphabet soup of preimplantation genetic testing is decoded in the video session with Dr. Shannon Clark and myself! Let’s start right there with defining all the terms:
The OLD TERMS, PGD (preimplantation genetic diagnosis) and PGS (preimplantation genetic screening), were used to describe screening for genes [PGD] versus chromosomes [PGD]. The new terms now are all based on PGT (preimplantation genetic testing).
PGT-M (old PGD) is to screen for particular genes. For instance if two members of a couple are carriers for Cystic Fibrosis this is the mechanism to screen the embryos. Watch the video to learn more about genetic carrier testing.
PGT-A (old PGS) is screening chromosome abnormalities such as trisomy 21 which causes Down syndrome. Chromosome abnormalities are increased with age of the women. We review in the video which scenarios are best to consider this strategy.
PGT-SR is a particular type of chromosome screening for couples who have chromosome rearrangements. Chromosome rearrangements are often identified in the context of couples with recurrent pregnancy loss.
Following an IVF cycle and retrieval, egg and sperm are fertilized and grown to a day 5 embryo or blastocyst. Unfortunately not all fertilized eggs become blastocyst embryos and not all blastocyst embryos make the criteria to undergo a biopsy. A biopsy of 3-5 cells is removed from the part of the embryo destine to become the placenta, also called the outer cell mass. The part of the embryo destine to become the child is called the inner cell mass. The biopsy is sent to a genetics company for the PGT testing. The remaining embryo is frozen in the IVF lab. Approximately 7-10 days later the results of the embryos are provided to the patient.
We discuss that PGT testing is not perfect and estimates are that when an embryo is identified as normal through PGT it is accurate about 97-99% of the time. The challenge is when embryos are identified as abnormal. The accuracy is lower in these embryos and up to 10-15% of embryos can be “overcalled”. This is due to the way in which embryos are biopsied. The goal with PGT is to assure that a normal embryo through PGT is truly normal. Some embryos have multiple cell lines represented and are called “mosaic”. Like the tile in our bathrooms, mosaic embryos have two cell lines that are identified with one being abnormal. We discuss in the video the challenges that exist regarding mosaic embryos.
I hope this provides an overview of the content that can be learned in our video discussion!