Mosaic embryos: a new opportunity in assisted reproduction

Are you familiar with mosaic embryos? Behind this intriguing name hides an important dilemma to consider in assisted reproduction. You will soon understand why. One of the greatest advances in reproductive medicine is the possibility of genetically analysing embryos before transfer, enabling us to detect healthy embryos with a higher probability of success. In this setting, it is also possible to detect mosaic embryos, which are embryos with a mixture of chromosomally normal and abnormal cells (for one or more chromosomes).

This anomaly is a source of concern for many patients. What does this diagnosis mean? Can these embryos result in a healthy baby, or should they be discarded? That is the dilemma. Thanks to advances in reproductive genetics, the answer is more promising than it was until recently.

What is embryonic mosaicism?

As mentioned above, embryonic mosaicism happens when an embryo has a mix of normal (euploid) cells and cells with chromosomal abnormalities (aneuploid). These abnormalities are caused by chromosomes not separating properly during cell division in the embryo, and they can’t be predicted or prevented.

Until a few years ago, it was believed that these embryos were not viable, but recent studies have shown that certain mosaic embryos can develop correctly and result in healthy babies. As a general rule, the presence of chromosomal abnormalities in an embryo usually prevents its implantation or causes a spontaneous miscarriage, as the embryo cannot develop further. Chromosomal abnormalities can also lead to certain pathologies that involve, for example, intellectual disability, developmental delay, heart disease, morphological abnormalities, etc. This is why mosaic embryos are usually discarded.

However, it is important to understand that the degree of mosaicism can vary. Some embryos have a low percentage of aneuploid cells, while others have a higher level of abnormalities. This difference is key to determining whether a mosaic embryo can be considered for transfer.

In fact, embryos diagnosed as mosaic have been shown to have the capacity to be implanted and result in a viable pregnancy. We must not forget that these embryos are classified based on the trophectoderm, which develops into the placenta, and not directly on the inner cell mass that will develop into the embryo. Research is still ongoing, but it appears that these mosaic embryos have the ability to
somehow correct the abnormal cells, resulting in a completely normal embryo.

PGT-A: the importance of embryo selection

Embryo mosaicism is detected through Preimplantation Genetic Testing (PGT-A), a technique that allows the genetic material of embryos obtained through in vitro fertilisation (IVF) treatment to be analysed. More specifically, we analyse the genetic content of cells biopsied from the trophectoderm, which is the outer layer of the embryo, using, in most cases, a highly advanced technology called mass sequencing.

Thanks to PGT-A, we can identify embryos with normal or altered chromosome sets. This helps select embryos with the highest probability of implantation, reducing the risk of miscarriage and improving treatment success rates.
In the case of mosaic embryos, PGT-A provides essential information to assess their development potential. Not all mosaic embryos should be discarded automatically, as some may implant and result in a viable pregnancy. The decision on whether to transfer them should be made based on individual medical criteria and the advice of reproductive genetics specialists.

When can we consider transferring a mosaic embryo?

The success of a mosaic embryo is difficult to predict, as these abnormalities, although identifiable, are not present in all cells and sometimes do not even affect development. Therefore, when deciding whether to transfer a mosaic embryo, we must consider the following:  

1. The absence of euploid embryos: When there are no chromosomally normal embryos available, mosaic embryos may be the only viable option to consider.
2. The degree of mosaicism: Embryos with a low percentage of aneuploid cells have a higher chance of success. Many laboratories already determine euploid embryos as those mosaics with less than 30% according to the recommendations of the Preimplantation Genetic Diagnosis International Society (PGDIS).
3. The type of chromosomal abnormality: Depending on the chromosomes affected and their location, these may have a more or less severe impact on embryonic development.
4. Medical history of the couple.

When is it not recommended?

According to the PGDIS, mosaic embryos with abnormalities in chromosomes 13, 15, 16, 18, 21, 22, X and Y are considered to be ‘high risk’ due to their high probability of resulting in a live birth with aneuploidy.

We must also discard and under no circumstances transfer so-called ‘aneuploid mosaic embryos’, as they have two or more different abnormal cell lines. This means that they are not viable and must be discarded along with aneuploid embryos.

Every embryo counts

Thanks to advances in reproductive genetics, we know that mosaic embryos should not be discarded automatically. The application of PGT-A at Tambre allows us to select the embryos with the highest potential, optimising implantation rates and reducing the risk of non-progressive pregnancy. 

In certain cases, when we do not obtain euploid embryos in IVF, we must consider and analyse each embryo because, in these circumstances, a mosaic embryo may represent a real opportunity to achieve the dream of becoming a mother. 

To date, of all the mosaic embryos that have been transferred worldwide, none have reported the birth of a child with problems associated with mosaicism that was diagnosed when it was an embryo. However, at Tambre, it is the medical team itself, together with the geneticist, who will be responsible for evaluating each case and each embryo in order to offer appropriate genetic counselling prior to transfer.

At Tambre, we have the most advanced technologies in embryo chromosome diagnosis in order to improve the chances of bringing a healthy baby home. We value each embryo because we know that each one is one more opportunity to achieve your dream of becoming a mother. For more information about mosaic embryos, preimplantation genetic diagnosis or other assisted reproduction techniques we offer at Tambre, contact us or call us on 91 411 61 11 to request your first consultation.