What is a Mosaic Embryo?

What is a Mosaic Embryo

Embarking on the journey of in-vitro fertilization (IVF) can feel like entering an entirely new realm filled with complex terms and procedures. One question that arises in the context of “mosaic embryo.” is what is a mosaic embryo? This article will delve deep into the nuances of mosaic embryos, their formation, commonality, and their potential implications for IVF outcomes.

Breaking Down the IVF Embryo Landscape

When IVF is opted for, it involves the examination of embryos through preimplantation genetic testing for aneuploidy (PGT-A). This testing process categorizes embryos into various types based on their chromosomal composition:

  1. Euploid Embryos: These are chromosomally normal embryos. They contain the exact number of 46 chromosomes and hence offer a higher probability of successful implantation and subsequent pregnancy.
  2. Aneuploid Embryos: These embryos have an abnormal number of chromosomes, which can be either more or less than the normal count of 46. Such embryos tend to have lower implantation rates and higher chances of being rejected by the uterus.
  3. Mosaic Embryos: These embryos fall into a kind of chromosomal grey area, as they possess a mix of normal (euploid) and abnormal (aneuploid) cells.
Euploid, Aneuploid and Mosaic Embryo

Digging Deeper: What is a Mosaic Embryo?

So, what is a mosaic embryo? A mosaic embryo is essentially one that consists of two types of cells – euploid and aneuploid. This means that within the same embryo, some cells have the correct number of chromosomes while others do not. It’s important to note that this state of mosaicism arises post-fertilization during the phases of cell division and growth of the embryo.

Unlike aneuploid embryos, where chromosomal abnormalities stem from either the sperm or the egg before fertilization, mosaic embryos’ chromosomal inconsistencies arise after fertilization. Technological advancements like Next-Generation Sequencing (NGS) have made it possible to identify such embryos with significant precision and reliability.

Prevalence of Mosaic Embryos

Mosaic embryos are not a new occurrence in the realm of reproductive science. It’s estimated that approximately 20% of all embryos are mosaics. This percentage may increase with maternal age due to declining egg quality and quantity.

NGS, a highly sensitive technique, is capable of quantifying the level of mosaicism in embryos. Depending on the proportion of abnormal cells present, embryos may be classified as low-level or high-level mosaics.

The Potential of Mosaic Embryos

The question that arises now is: Can mosaic embryos develop into healthy babies? The answer isn’t straightforward due to the diverse outcomes possible with mosaic embryos.

Mistakes can occur during the rapid cell division following fertilization, leading to the formation of abnormal cells. However, if these cells die off or are confined to the placenta, a healthy baby can still be born. This phenomenon is known as “self-correction.”

On the flip side, if the abnormal cells multiply within the embryo, it could lead to potential complications like implantation failure, miscarriage, or birth defects.

Despite the uncertainties, some studies suggest that mosaic embryos that progress to full-term pregnancies have similar odds of being born without any noticeable genetic differences compared to normal embryos.

Deciding on Mosaic Embryo Transfer

Deciding whether or not to transfer a mosaic embryo is a deeply personal decision. It requires careful consideration and consultation with medical experts.

In situations where no euploid embryos remain, the potential transfer of a low-level mosaic embryo may be considered. This decision is often taken after extensive consultation with a genetic counselor and a fertility specialist.

High-level mosaic or abnormal embryos are generally not preferred for transfer due to their increased risk of potential birth disorders, miscarriage, and lower implantation rate. However, in special circumstances, a low-level mosaic embryo transfer may be considered.

Final Thoughts

The realm of IVF and genetic testing is complex and constantly evolving. While the question, “what is a mosaic embryo?” has been answered, each case is unique and requires personalized medical guidance.

It’s crucial for patients to have open discussions with their medical teams about the potential implications of mosaic embryos. Knowledge is power, and understanding each aspect of the IVF process can help patients make informed decisions and navigate their fertility journey with greater confidence.


  1. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8715143/