The Alphabet Soup of PGT PGD PGS

In the past 30 years, Embryo biopsy for genetic testing has evolved in extraordinary ways.  Embryo biopsy is used to assess for chromosomal normalcy and/or severe genetic diseases; this  has been a revolutionary accomplishment. The technique is performed by highly skilled embryologists using a laser microscope to make a pinhole opening in the shell of the embryo, that will permit the passage of a few cells for removal and analysis.  It is breathtaking to watch on the microscope monitor. It is one of the most technologically complex tests in all of medicine. The cells removed during this micromanipulation embryo biopsy are cells destined to become the placenta. Care is taken not to disrupt the cells destined to become the fetus.

The next step is to freeze the embryo while the DNA testing can be done on the cells and a report can be received to guide the selection of the healthiest embryo for transfer. The DNA testing is also highly sophisticated. There are so few cells for analysis, that the first step is to replicate the DNA. The next step is to digest the DNA into fragments that are representative of each of the 23 pairs of chromosomes. A perfect set of 23 pairs of chromosomes is the basis of normal implantation. Chromosomal imbalances can occur during egg maturation and at the time of fertilization. Such chromosomal errors are related to the age of the egg and are a part of human biology. These chromosomal imbalances are the main cause of miscarriage and or failed implantation.  A lab that specializes in the DNA testing of embryonic cells is utilized. If the patient(s) have a proven risk of a severe genetic disease, the DNA fragments can be processed to reveal that specific gene to determine if the normal gene or if an abnormal gene variant is present. 

So, why are there so many initials for the same thing? As technology evolves so does the nomenclature. Currently, PGT is for Pre-Implantation Genetic Testing.  There are three types of such testing: PGT-A for Aneuploidy; PGT-M for Monogenic disease; PGT-SR for structural rearrangement of chromosomes. “Aneuploidy” translates to “wrong number” and specifically identifies whether there are 23 pairs of chromosomes or not. While each of these tests requires IVF to create embryos, and embryo biopsy to release some cells for testing, and freezing of embryos to await results, each DNA test is technologically different. PGT-M and PGT-SR require test creation specific to the patients involved; and guidance by genetic counselors and physicians to identify the genetic variants that put these patients at reproductive risk.

The accuracy of the current technologies of PGT-A is approximately 98%, and PGT-M is approximately 95%. This is extraordinary given that we are working with so few cells, and we are working with placental cells. But it is still not 100%. The patients are still advised to assess the pregnancy with ultrasound, and bloodwork, and decide with their obstetricians and geneticists regarding amniocentesis or chorionic villus sampling. 

It is phenomenal that we have the technology to assess for chromosomal normalcy of the embryo prior to transfer and implantation. This technology has permitted a significant reduction of severe genetic diseases; a reduction of the miscarriage rate related to aneuploidy; a commitment to single embryo transfers and a reduction of the twin rate of IVF; and therefore a reduction of the known obstetrical complications that occur at a higher rate with twins; and, for women over 35, an increase in the overall success rates for IVF.  

As PGT is designed to test for chromosomal imbalances, there will be less embryos available for transfer which can be disappointing for patients. However, the embryo(s) that are available for transfer are of the best quality. “Priceless”.  Also, there are concerns about over diagnosing or under diagnosing. This is true for every test in all of Medicine; and is being addressed with the most current platform for PGT that utilizes a large data bank as additional artificial intelligence. There are also concerns about mosaic embryos, specifically an embryo that has some normal cells and some abnormal cells.  These embryos can still be transferred as they do provide for pregnancies but at a much lower rate. The patients need to make their best decisions regarding mosaic embryos with the assistance of knowledgeable physicians and geneticists. As we have only recently been able to identify mosaic embryos, and as most patients can have a successful pregnancy without transferring mosaic embryos, we are still in the early phases of gathering data on pregnancy, miscarriage and the birth defect rates of such embryos. Ultimately, PGT is a valuable option for patients doing IVF and for patients with known risk of genetic diseases. With the support of the physicians, geneticists, and embryologists, Knowledge is Power. 

TEBX from Maureen Moomjy on Vimeo.

Maureen O'Brien Moomjy, MD, FACOG

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