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Mosaic Embryos

As preimplantation genetic testing has advanced over the years, new classifications of embryos have occured, including mosaic embryos

With the increased use of PGT-A (preimplantation genetic testing for aneuploidy) in IVF protocols, REIs and their patients have more information than ever about the genetic makeup of embryos. Having more genetic information helps the fertility team decide which embryo is best to transfer during an IVF cycle. Genetic testing can also tell you whether any embryos are classified mosaic embryos.

PGT-A samples a few cells from the placental layer of the embryo and tests their chromosomal makeup. Up until a few years ago, PGT-A technology could only provide two results – normal or abnormal. In scientific language, normal embryos that contain the correct number of chromosomes are called euploid. Abnormal, or aneuploid embryos, do not have the correct number.

Aneuploid embryos are the main cause for failed implantation in IVF. Therefore, the ability to test and choose embryos that are euploid allows for the selection of the best embryos for transfer and hastens the time to that positive pregnancy test!

But as PGT testing has advanced over the past few years, it reveals that not all is black and white with human biology. Indeed, we are finding that some embryos are not one or the other, but can fall on a spectrum between euploid and aneuploid with some having a mix of normal and abnormal cells. This is called mosaicism.

Embryos that are mosaic can have different proportions of normal and abnormal cells. A low-level mosaic embryo would have mostly normal cells and a lower percentage of abnormal cells. A high-level mosaic embryo would have mostly abnormal cells and a lower percentage of normal cells.

An important point to make is that mosaicism happens after fertilization, that is after the egg and sperm meet. Frank chromosomal abnormalities, such as Down’s syndrome, are due to abnormal egg or sperm development and are, therefore, present prior to fertilization. For this reason, mosaicism does not increase with the parents’ ages and occurs equally in all age groups.

Geneticists have a criteria for mosaicism

Geneticists who work in this field have established the following criteria to label embryos tested with PGT-A:

  • Normal – fewer than 20 percent of the cells in the embryo are abnormal
  • Low-level mosaic – 20 to 40 percent of the cells are abnormal
  • High-level mosaic – 40 to 80 percent of the cells are abnormal
  • Abnormal – greater than 80 percent of the cells are abnormal

Deciding whether to transfer mosaic embryos

PGT-A is not an exact science. It is only a tool to help patients and physicians choose which embryos to transfer. Therefore, if you have mosaic and abnormal embryos, an important discussion between you and your doctor is whether they are safe to transfer. The concern, of course, is whether transferring these embryos could lead to living children with significant congenital health issues.

Physicians typically are unwilling to knowingly transfer embryos with abnormal test results. However, some reports indicate that it may be possible for some embryos with abnormal test results to result in live births of apparently healthy children. Therefore, even given the possible risks, some patients with only mosaic or abnormal embryos might decide these known or unknown risks are acceptable in their pursuit of parenthood and may choose to transfer these mosaic or abnormal embryos. When patients consider transferring mosaic embryos, we provide them with access to genetic counselors to help explain their individual risks based on the specific type of abnormality.