The Role of Preimplantation Genetic Screening in IVF

Over the past 2 decades, preimplantation genetic screening (PGS) has fallen in and out of favor among fertility specialists.  The benefits of performing PGS with in vitro fertilization have been hugely debated.  While the techniques and technology for PGS have advanced during this time, it’s still unclear whether PGS provides a benefit and that should be determined for each individual in consultation with their doctor.

When PGS was first introduced in the late 1990’s as an option for testing embryos to see whether or not they are chromosomally normal, it was deemed a revolutionary technology that would change the way IVF was performed.  At that time, embryos were biopsied for PGS three days after fertilization, during the cleavage stage.  In the cleavage stage, each embryo contains approximately 6-8 cells and 1-2 of those cells were removed during the biopsy. Many fertility practices started utilizing this technology, but in 2007, a report published in the New England Journal of Medicine showed that PGS actually decreased pregnancy rates in women over 35 years old.  Based on this report and others, the American Society of Reproductive Medicine in 2008 stated that PGS was ineffective.

PGS at that point was largely abandoned until new technology for day 5, blastocyst stage, biopsy was introduced in approximately 2010.  At the blastocyst stage, an embyro contains hundreds of cells and is less fragile. So, removing approximately 5 cells for PGS testing at that point was thought to be less harmful to the embryos.  A PGS revival ensued and it once again became a heavily utilized technology.  PGS helped physicians choose the best embryos to transfer since they were able to select the embryos that were chromosomally normal.  PGS also helped reduce the risk of twins, triplets,and high order multiple births.  By being able to select chromosomally normal embryos for transfer, fewer embryos could be transferred and still give a high success rate.

Embryos deemed abnormal through PGS were traditionally discarded. What we are starting to learn though is that all of those embryos may not actually be genetically abnormal.  There have been several reports now of genetically normal babies being born after the transfer of PGS abnormal embryos.  To understand how this might happen, we have to go back to the biopsy stage.  Only approximately 5 cells are tested from an embryo that contains hundreds of cells.  We know that embryos have the ability to correct some DNA errors, so maybe those cells that were deemed abnormal would have gone on to correct themselves later in the embryo.  In addition, the cells that are biopsied for PGS testing come from an area of the embryo that eventually becomes the placenta.  Maybe the embryos are segregating those cells out to the placenta for a reason.  So, what this means is that we may be calling some embryos abnormal when they could actually result in healthy pregnancies.

In my opinion, there is a time and place for PGS.  I don’t recommend it to all patients doing IVF.  It has to be on a case by case basis.  For instance, for a young woman who has many embryos, it may help with selecting a single embryo for transfer and reducing her risk of multiples.  Or, for a couple who has had multiple miscarriages, it may help prevent another miscarriage due to aneuploidy.

The decision to do or not do PGS should be made based on a frank discussion between a patient and their physician.  If PGS testing is something you’re considering doing with your IVF cycle, it’s important to talk to your physician about the risks versus benefits.

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