IVF Babble

What is a mosaic embryo?

There is always so much to take in when trying to understand IVF. Just when you thought you had it sussed, new terminology comes along and you find yourself having to ask once again “sorry, can you explain?!”

As confusing and frustrating as this can be, what it does mean is that science is advancing and therefore embryologists are learning more about ways to enhance your chances of a successful pregnancy.

Following studies by the world’s largest genetic laboratory, we now have the term ‘mosaic embryo’ to get to grips with. Although not a brand-new discovery, it is still a revelation that is changing the way embryologists deal with embryos that have chromosomal abnormalities.

Up until now, we had only ever focused on embryos that are either normal (euploid) or abnormal (aneuploid), so now we need to understand this third category.

We turned to Chara Oraiopoulou BSc, MRes Biologist, Clinical Embryologist from Embryolab Fertility Clinic in Greece and asked her to explain.

Can you explain what a mosaic embryo is?

A mosaic embryo is an embryo that contains both euploid cells (cells with the correct number of chromosomes) and aneuploid cells (cells with incorrect number of chromosomes).

How do you find out if you have a mosaic embryo?

During the last few years, preimplantation genetic testing for aneuploidies (PGT-A) with the method of next generation sequencing (NGS) provides reliable detection of embryo mosaicism. The procedure involves a biopsy of the embryo, routinely performed on day 5, cryopreservation of the biopsied embryo and genetic analysis of the cells obtained.

 Can you explain what PGS testing is?

PGS (preimplantation genetic screening) is nowadays known as PGT-A (preimplantation genetic testing for aneuploidies). PGT-A is a genetic test that detects embryos with the correct number of chromosomes (also called euploid). In this way, the transfer of aneuploid embryos (incorrect number of chromosomes) into the uterus is avoided, increasing the chances for a healthy and viable pregnancy. The introduction of a new technology called NGS for PGT-A has provided information about mosaic embryos as well.

How many chromosomes should a healthy embryo have?

A healthy embryo should have 46 chromosomes in each cell. Half of them are contributed by the sperm, while the other half is contained into the oocyte.

 What happens if you have a mosaic embryo? If it means your embryo has some abnormal cells, surely you don’t transfer it?

During a blastocyst (day 5 embryo) biopsy, typically 4-8 cells are removed from the embryo and sent for genetic analysis. These cells are removed from the part of the embryo called trophectoderm, that is destined to form the placenta. Cells cannot be removed from the part of the embryo that will form the fetus, called inner cell mass (ICM). Mosaicism could be detected throughout the embryo or be eliminated in a part of it. So, a possible scenario could be that the biopsied sample is mosaic, but the inner cell mass that will form the baby is euploid. For that reason, mosaic embryos are not excluded, but are potentially transferable if no euploid embryos are available.

Recent guidelines suggest prioritisation of mosaic embryos, according to the percentage of aneuploidy in the mosaic sample. Also, published data shows that mosaic embryos can lead to healthy pregnancies, but at a lower percentage compared to euploid embryos. Therefore, it is not advised to exclude mosaic embryos from transfer, as it could lead to embryo wastage.

What happens if you do transfer it? What are the risks?

Most likely, mosaic embryos have been transferred for years before the development of NGS technology. Although there are not yet detailed studies on the follow-up of transferred mosaic embryos, it was shown that miscarriage rate is higher and implantation rate is lower. Genetic counselling before embryo transfer as well as prenatal diagnosis by amniocentesis are recommended.

How common is to have a mosaic embryo?

There is variability between studies concerning the incidence of mosaicism in blastocyst-stage embryos, with the most recent estimates range between 4% and 22%. This variation could be caused by either biological factors, such as embryo-culture conditions in the IVF laboratory, or technical factors, as lack of standardised systems for PGT-A result interpretation.

Huge thanks to Chara Oraiopoulou BSc, MRes Biologist, Clinical Embryologist from Embryolab Fertility Clinic in Greece.



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