The phrase “embryo testing” is most commonly used to describe Preimplantation Genetic Testing (PGT). PGT is the process by which 5-8 cells from the embryo, from the point at which it becomes a more developed embryo, called a “blastocyst,” are removed and analyzed to understand more about of the genetic makeup of the remaining cells within the embryo, therefore giving doctors more information about the genetics of the resulting fetus.

What Genetic Testing Can Tell You
Although there are several things we can test the embryo for, patients most commonly undergo PGT-A (aneuploidy). In PGT-A testing, the cells that are analyzed are evaluated for the number of chromosomes they contain. Typically, most developmentally ‘normal’ males and females have 46 chromosomes within their cells (male are 46XY and females are 46XX), termed euploid.

Studies have shown that as women age, the percentage of embryos that are made from eggs that are chromosomally normal decreases, with a sharp decrease in percentages in the late thirties and early forties. The majority of chromosomally abnormal embryos, or aneuploid embryos, will either lead to a negative pregnancy test or miscarriage. Patients elect to pursue PGT-A testing in order to identify which embryo are 46XX vs 46XY, in order to select the embryo to transfer that is most likely to result in a live birth.

Current data shows that, although PGT-A is useful and accurate in patients under 35 years of age, it does not improve their overall outcomes. While the process of PGT-A helps to select the best embryo for transfer, it does not change anything about the embryo itself.

Then, “Why do it?” you might ask.

Why Do Genetic Testing
Well, for starters, even if a woman is young and seemingly healthy (for example, an egg donor), they can still produce eggs that lead to embryos that are chromosomally abnormal. Although the percentage of embryos that are normal are likely to be much higher in younger women, when choosing an embryo to transfer, it is possible that although the embryo looks morphologically normal, that it could be aneuploid—and therefore not likely to lead to a live birth.

These are just a few examples where we find PGT-A testing is helpful:

  1. In younger patients, PGT-A testing is a way to avoid transferring embryos that are not going to get them pregnant, and work more efficiently in optimizing success in a shorter period of time.
  2. It is also helpful for patients who have multiple embryos remaining after a successful transfer by identifying which of those embryos, if any, are chromosomally normal. Patients then have a better sense of successful outcomes with those embryos when they return in the (near or distant) future to have more children.
  3. PGT-A testing allows us to identify the biological gender of the embryos, if that information is important to patients. Patients may elect to ‘balance’ their families and choose an embryo of a particular biological gender as a result.

Mosaic Embryos
A newer topic involving PGT-A in the last several years is mosaicism. The term ‘mosaicism’ refers to a single embryo that might contain cells that are chromosomally normal and abnormal. As technology advances, and the level of detail we are able to evaluate increases, distinguishing whether these mosaic results are in fact true, or artifacts of the highly detailed testing modality, can make things seem more complicated.

However, over the last several years, we have observed the outcomes, as these embryos have been (cautiously) transferred, and have found that—although lower implantation and live birth rates, as well as higher miscarriage rates have been noted—many chromosomally normal live births have occured. Although careful counseling and selection of the mosaic embryo must be done prior to implantation, it is possible to have a successful outcome and healthy baby from these embryos as well.

Overall, many patients elect to do PGT-A when undergoing an IVF cycle. It may not be the right choice for everyone, but for many patients, there are several benefits.

Nidhee Sachdev, MD has trained among the most prestigious and diverse medical programs in the country, including fellowship training in reproductive endocrinology and infertility at the prestigious New York University (NYU) Langone Fertility Center in New York City where she conducted research on preimplantation genetic screening (PGS) and the University of Chicago Medical Center, where she earned the academic distinction of chief resident in obstetrics and gynecology, and trained under a top recurrent pregnancy loss expert. Dr. Sachdev is passionate about providing individualized, collaborative patient care. She started her medical career in Orange County, earning her Doctor of Medicine at the University of California, Irvine, School of Medicine.