The term “mosaic” is assigned to an embryo that is found to have both normal and abnormal cells during PGS testing at the 5 day blastocyst stage. In this form of testing, doctors are able to look for and detect, with a high amount of accuracy, the presence or absence of certain genes that may lead to developmental concerns later in pregnancy. For example, tests are often able to detect the presence of an additional copy of chromosome 21, a condition that leads to Downs Syndrome.
In IVF, these tests are generally performed in an effort to transfer only the best quality embryos into an awaiting uterus. With the high costs of IVF treatments and the fact that donor egg cycles often result in more viable embryos than needed, many couples choose to test for abnormalities and then discard any embryos that suggest less-than-optimal chances for implantation.
Despite this common practice, couples are beginning to consider using these “mosaic” embryos. The use of mosaic embryos has been shown to show some promising results for normal pregnancies and healthy children upon birth, which is especially important in cases when the gender of the child is an important factor to the couple,.
A 2016 article published by The New York Times reported 10 healthy births resulting from mosaic embryos. The study also showed a 40% success rate of pregnancy in IVF attempts made with mosaic embryos.
During early cell division, many of the flawed genetic strains that would result in an abnormal pregnancy or birth defect can be lost, or segregated and stored in the placenta. If this were to happen, as opposed to the abnormal cells being stored in the genetic make-up of the fetus, the growing embryo would, in a manner of speaking, be able to “self-correct.”
The alternative would be that the abnormal cells do enter the genetic make-up of the embryo. In these cases, experts suspect that the most likely scenario is that the flawed embryo would fail to implant, or result in a first trimester miscarriage.
It is important to note that the study of mosaic embryos is only in its infancy, and there are no current long-term studies on potential defects that may surface later in life. The prevailing conclusion is that one should be very, very thoughtful when considering the use of mosaic embryos and prepared for any circumstance that may arise as the result of their use. If you have specific questions about mosaic embryos as they relate to your specific needs, you should be sure to speak with your IVF doctor well in advance of your IVF cycle.