How Mosaicism is Rewriting the Classic PGS Tale

By Shannon Wieloch — April 24, 2018

2 min read

I love fractured fairy tales.

I assure you, it’s a thing. Think Wicked by Gregory Macguire (though my personal favorites are the Insanity Series by Cameron Jace and The Land of Stories by Chris Colfer).

The premise is to take a story you know and change the characters, setting, point of view, or plot. The story may occur  in a dystopian America 500 years in the future, or begin in the Evil Queen’s childhood, or perhaps be the retelling of your own country’s history… set to hip-hop.

In essence, it’s flipping the script. This concept of flipping the script happened to be the theme for this year’s National Infertility Awareness Week. We were urged to change the conversation around trying to conceive and support the 1 in 8 who are impacted by infertility.

In thinking about this theme, it was obvious to me that the scripts of our lives are constantly being rewritten, be it on the political grandstand or in the everyday minutia of our lives. As a genetic counselor, when I heard the phrase “flip the script”, I immediately thought of my PGS patients. PGS, or preimplantation genetic screening, is a genetic test performed on IVF embryos with the goal of helping improve the chance of achieving a successful pregnancy. PGS was initially designed to tell patients and doctors if an embryo was normal (euploid) or abnormal (aneuploid), giving them more information about which embryo to transfer. However, advancements in PGS technology have flipped this script, adding in a new character, a new possible result: mosaicism.

Stick with me here:

The Old Script (i.e., The Taming of the Shrew):

The fertility patient undergoes IVF, and samples from her embryos are sent to a laboratory for PGS testing.  In this script there are only two main characters, two possible results: Euploid and Aneuploid. The story is relatively predictable; the audience always roots for Euploid.

The Flipped Script (i.e., 10 Things I Hate About You):

The fertility patient undergoes IVF, and samples from her embryos are sent to a laboratory for PGS testing, this time, with the latest technology. In this script, a new character, Mosaicism, who has been waiting in the wings, always there but never before seen, is introduced, which results in entirely new, and unpredictable, plot twists.

You see, Mosaicism is the newest character in the PGS saga. And while everyone is still rooting for Euploid (because these embryos are known to have the best chance of success), the appearance and ensuing curveballs that come with Mosaicism are a very real possibility for any couple undergoing PGS.

Mosaicism isn’t the bad guy, but he’s not guaranteed to be the hero either. Mosaic samples contain both normal euploid cells and abnormal aneuploid cells. These embryos are shown to have lower success rates than euploid embryos, but they can still lead to a healthy baby.

“To transfer or not to transfer, that is the question.” Okay, I’ve crossed the line by mixing Shakespearean plays, but regardless, the sentiment is true. So what’s the audience, i.e. you the patient, to do, in the event that a Mosaic is the only character in your story, and there are no euploid embryos available?

I suggest that you do your research. Mosaicism has been around forever. If you think about it, you or I may have developed from a mosaic embryo. Yet our ability to detect mosaicism is new. There are a lot of possible outcomes after transfering a mosaic embryo – non-implantation, miscarriage, the birth of a healthy baby, or the birth of a child with special needs. Discuss your situation with your fertility doctor, a genetic counselor, and other health professionals. We are not here to tell you what to do but rather to be your sounding board; to play out each scenario with you, to validate or correct the endless amount of information out there, and to help you determine what level of risk you are comfortable with taking.

I once heard it said that we are currently writing our own history on this topic. The healthy babies that have been born from the transfer of a mosaic embryo are still so young, and while long term follow up doesn’t exist yet, we are actively engaging in this research. We are, without a doubt, only in the prologue of this story. There’s still writing, editing, revising, and proofreading to be done, but we know our goal– to have an audience cheering at the end of the show.

In other words, we want all of our patients to have a fairy tale ending to their family building journey.