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Preimplantation Genetic Testing (PGT)

Many couples pursuing In Vitro Fertilization (IVF) ask if preimplantation genetic testing (PGT) will increase their chances of having a baby.

The short answer is, “No”.  Genetic testing does NOT increase the chance that the IVF treatment cycle will produce a baby.  What it can do is increase the chance that an embryo transfer will work.  This, however, assumes that there is a normal embryo to transfer.  A more comprehensive explanation for this is below.  But before getting into the specifics, it is important to understand some of the terminology that we use as it pertains to IVF and genetic testing.  

An embryo is a fertilized egg.  Not all eggs will fertilize, and not all fertilized eggs (now called embryos) will develop into a blastocyst.  A blastocyst is an embryo that has met certain developmental milestones which increase the likelihood that an embryo transfer will result in implantation, a necessary step to get to the outcome of interest…a baby.   

Even an embryo with normal (“euploid”) genetic testing does NOT guarantee a live birth.  Across the country, the national average for ongoing pregnancy rates (that is heart beat on Ultrasound at approximately 8 weeks gestation) with euploid elective Single Embryo Transfer (eSET) is about 60-70%.  There are multiple reasons why the live birth rate is not 100% which is outside of the scope of this blog.

So, let’s get back to the basics.  There are two types of preimplantation genetic testing:

● Preimplantation genetic testing for aneuploidy (PGT-A) (formerly called preimplantation genetic screening) is performed on approximately 3-5 cell(s) removed from a blastocyst embryo. The goal of PGT-A is to identify chromosome imbalances in embryo(s) of couples who are presumed to be chromosomally normal.   (ie. 46,XX being female and 46,XY being male). Theoretically, avoiding the transfer of embryos which are not euploid after PGT-A testing, reduces the risk of pregnancy failure and improves the probability of conceiving a viable pregnancy with each embryo transfer performed.  However, if the cycle does not produce a euploid embryo, there will be no embryo transfer, and in that case the genetic testing did NOT increase your chance of having a baby—it does not change the embryo!

● Preimplantation genetic testing for monogenic (single-gene) disorders (PGT-M) is also performed on cell(s) removed from a blastocyst embryo. The goal here is to establish a pregnancy that is unaffected by a specific genetic condition(s), such as a known heritable genetic mutation carried by one or both biological parents. This includes conditions like Cystic Fibrosis or Spinal Muscular Atrophy.  Keep in mind that genes are the smallest unit of the DNA code.  Just like letters make up a word and words tell a story so too do “letters” make up single genes which code for proteins involved in human development.    So, essentially looking for a single gene disorder is like looking for a specific word in a novel with over 20,000 unique words.  Furthermore, the “coding” sequences make up only a small fraction (<2%) of your DNA.  In short, PGT-M is like looking for a needle in a haystack and this genetic testing on embryos requires special probes to be made by a specialty genetics lab.

There are benefits to PGT-A, particularly in the setting of IVF cycles in women of advanced reproductive age (i.e. > 38 years old, some programs say > 35 years old).  The main benefit of PGT-A is to avoid the transfer of an abnormal embryo that will not produce a baby, or maybe worse, could possibly produce a baby with a birth defect due to a chromosome imbalance (i.e. Down Syndrome).

In general, I do NOT recommend PGT-A in egg donation cycles because there is only limited data to support this treatment option for “younger” eggs. The American Society for Reproductive Medicine (ASRM), who sets the standard of care in our field, recently addressed this issue (March, 2018) making a summary statement that, ” The value of PGT-A as a universal screening test for all IVF patients has yet to be determined. Some studies…(do suggest)… that in the primary embryo transfer of favorable-prognosis patients, the potential for this testing to increase eSET utilization and further decrease the incidence of multiple gestations. ”

In summary, the overall goal in our field is to help couples have a baby with Elective Single Embryo Transfer (eSET) while minimizing harm/risk that may result from with multiple pregnancies.  There is no promise or guarantee on the health of such child, because this is NOT what PGT-A is intended to test.  

What are Limitations of PGT-A:

  • PTG-A does NOT test for single gene disorders. (i.e. Cystic Fibrosis)
  • PGT-A does NOT test for chromosomal rearrangements (i.e. Translocations)
  • PGT-A does NOT test for triploidy (i.e. 69, XXX or 69, XXY)
  • PGT-A does not test for Epigenetic and Imprinting Disorders.  These are genetic conditions that result from errors in the “on” and “off” switches for the gene in question, not errors in the DNA sequence. (For more information on Epigenetics go to https://www.whatisepigenetics.com/fundamentals/)
  • PGT-A is NOT a screen for birth defects.  
    • Keep in mind that embryo development is a dynamic and ongoing process.  Birth defects may result from developmental errors not due to genes at all.  The most obvious example would be fetal alcohol syndrome due to alcohol exposure which causes mental retardation and birth defects and is not due to a gene mutation at all.  
  • No test is perfect.  PGT-A  test accuracy is 98%.  A “false abnormal” test result may lead one to discard what is actually a normal embryo.  Conversely, a “false normal result” may lead to the transfer of an abnormal embryo. 
  • PGT-A may reveal no normal embryos are available for transfer
  • PGT-A may not produce results because the DNA from the removed cells has degraded.

In summary, PGT-A does NOT increase the cumulative chance of a live birth (i.e. the live birth rate from a treatment cycle after all of the eggs are thawed and all of the resulting embryos are transferred). This is because if the treatment cycle does produce a normal embryo that is destined to work, the genetic testing does NOT change that fact.  However, the genetic (PGT-A) testing does increase “implantation” rates because we are putting back less “bad embryos”, the ones we know are  NOT destined to work.  

Always consult with your doctor for questions regarding this testing, and understand that if your physician feels that more teaching/counseling is needed prior to treatment a formal Genetics Consultation may be required.

Teresa M. Erb, M.D.

Florida Institute for Reproductive Medicine proudly serving patients in North Florida and South Georgia including Savannah, Columbus, Jacksonville, Jacksonville Beach, Orange Park, St. Marys, Palm Coast, St. Augustine, Palatka, Lake City, Tallahassee, Daytona, Ponte Vedra, Gainesville, Orlando, Melbourne, Ormond Beach, New Smyrna, Port Orange, Brunswick, St. Simons Island, Thomasville, Tifton, Albany, Brunswick, Valdosta and Dothan Alabama.

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