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Writer's pictureKimberly Schamroth, RN

Mosaic Embryos: What They Mean for IVF and Your Fertility Journey

When undergoing in-vitro fertilization (IVF), couples often encounter a wide range of terms and concepts that can seem complex and daunting. One such term is "mosaic embryos." If you are on a fertility journey, it is essential to understand what mosaic embryos are, how they differ from normal embryos, and what their presence means for your chances at a successful pregnancy. In this blog post, we will explore the topic of mosaic embryos In detail to help you become a more informed patient and advocate for your care.


What is a Mosaic Embryo?

A mosaic embryo is am embryo that contains a mix of normal and abnormal cells in terms of their genetic make up. In a mosaic embryo, some cells have the correct number of chromosomes (46 total, 23 from each parent cell), while others have an abnormal number (less or more than 46). This is known as "mosaicism."


There are a few terms you may hear associated with this such as "euploid" or "aneuploid." An euploid embryo is an embryo with the correct number of chromosomes, 46, while an aneuploid embryo has an abnormal number (more or less than 46).

image shows different embryo representations
An representation of euploid, mosaic, and aneuploid embryos

Mosaicism can occur during early embryonic development when an error happens in cell division. For example, a cell may begin as euploid, but as it continues to develop and divide, some of the cells may undergo abnormal chromosomal divisions, leading to a mix of normal and abnormal cells.


How are Mosaic Embryos Identified?

Mosaic embryos are typically identified through a process called Preimplantation Genetic Testing for Aneuploidy (PGT-A). During this test, a few cells are removed from the embryo at the blastocyst stage, usually around day 5 or 6 after fertilization. These cells are analyzed to determine their chromosomal makeup.


The results of the test lead to the categories of:

  • euploid- cells with the correct number of chromosomes
  • aneuploid- cells with an abnormal number of chromosomes which usually leads to a failed implantation, miscarriage, or a genetic disorder
  • mosaic embryos- a mix of euploid and aneuploid cells

What Does it Mean to have a Mosaic Embryo?

The presence of mosaic embryos can be confusion and emotionally challenging for couples. The implications of transferring a mosaic embryo depend on several factors, including the percentage of abnormal cells and which chromosomes are affected.


Chances of a Successful Pregnancy

Research shows that mosaic embryos have a lower implantation rate compared to fully euploid embryos (1). However, they do have the potential for a successful pregnancy, particularly when the percentage of abnormal cells is low or when certain less critical chromosomes are affected. Today, fertility specialists may suggest additional genetic screenings for birth defects throughout your pregnancy if you transfer a mosaic embryo, although at this time evidence-based guidance is lacking.


An amniocentesis, a procedure used to take a small sample of amniotic fluid from the uterus to test cells directly from the fetus is the gold standard for prenatal follow up tests (2). This fluid surrounds the fetus during pregnancy and can provide powerful information about the baby's health. Remember, it is important to discuss with your doctors the risks associated with amniocentesis.


Risk of Miscarriage or Genetic Disorders

The main cause of failed pregnancy or a miscarriage in both natural conception and assisted reproductive technology is an aneuploid embryo (3). This occurs at the first stage of fertilization, known as meiosis. Even if an embryo starts off as genetically euploid, as it continues to multiply, divide, and grow, there is a risk for abnormalities that rise to a mosaic embryo. "Compared to euploid embryos, mosaic embryo transfer (MET) has also been shown to be associated with increased risk of miscarriage in multiple studies" (4).


Should You Consider Transferring a Mosaic Embryo?

Deciding whether to transfer a mosaic embryo is a deeply personal decision and should be made in consultation with your fertility specialists and genetic counselor. To help you with this decision, here are a few things you could consider when making a decision:


  1. How many available embryos are there?

    If you only have mosaic embryos available or if you have a few euploid embryos, you might consider transferring a mosaic embryo, especially if the mosaicism involves a low percentage of abnormal cells.

  2. Type of Chromosomal Abnormality

    Some chromosomal abnormalities are less likely to result in severe outcomes than others. Your fertility clinic and genetic counselor can provide more detailed information on the specific abnormalities detected.

  3. Personal and Ethical Considerations

    Some individuals and couples may feel more comfortable only transferring embryos with a higher chance of being chromosomally normal. Others may be willing to take the risk if mosaic embryos are the best option of achieving a pregnancy. Take the time to talk through this important component.

  4. Cost and Availability of PGT-A

    The cost of PGT-A and the potential need for additional IVF cycles if mosaic embryos are not transferred can also play a role in the decision-making process. Make sure to talk to your fertility care team and try to determine what you can afford, what may be covered, and your chances of needing to do additional cycles.


What Does the Future Hold for Mosaic Embryos?

The field of reproductive medicine is quickly evolving and research into mosaic embryos is ongoing. New studies are helping us understand more about the potential outcomes of transferring mosaic embryos and this knowledge may lead us to more refined guidelines and better predictions of success.


Currently, many clinics are cautiously optimistic about the potential of mosaic embryos. As testing methods improve and more data becomes available, the ability to counsel patients on the risk and benefits of transferring mosaic embryos will become even more precise.


We Are Here to Help

The discovery of a mosaic embryo can be both hopeful and a challenging moment in your fertility journey. It is important to stay informed, ask questions, and work closely with your medical team to make the best decision for you. Every single fertility journey is unique and with the right support and information, you can make choices that align with your values, needs, and goals.


If you want someone to help you better understand your options, assist in creating questions or resources so you feel more confident in your understanding of your embryos, or simply want someone to talk to you, Grain Fertility Patient Empowerment Coaches are here to answer your questions.


If you are a new member, simply go to www.grainfertility.com/signup to create your account and schedule an initial consultation appointment with one of our Empowerment Coaches. If you are using the free version of our app and want to take advantage of our Premium Member benefits including custom guides and one on one coaching, you can go to your Account to upgrade your service today and begin meeting with our coaches regularly.


Citations

  1. Greco E., Greco PF, Listorti I, et al. The Mosaic Embryo: what it means for the doctor and the patient.

  2. Besser, Andria G et. al. Evidence-based management of preimplantation chromosomal mosaicism: lessons from the clinic

  3. Hassold T., Hunt P. To err (meotically) is human: the genesis of human aneuploidy

  4. Munne S., Spinella F., Grifo J., Zhang J., Beltran M.P., Fagouli E. Fiorentino F. Clinical outcomes after the transfer of blastocysts characterized as mosaic by high resolution Next Generation Screening- further insights.


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