Mosaic embryos – to transfer or not to transfer?

I am a gynaecologist by training and have been working in fertility medicine for over fifteen years, with more than twelve years of exclusive focus on reproductive medicine. I studied medicine in Peru, then trained in fertility medicine in Barcelona, before moving to Ireland, where I worked for over eight years in both clinical and management roles. I recently returned to Barcelona, where I now work as a fertility consultant at Institut Marquès and also hold a medical director role for Western Europe. My work is primarily clinical, supported by years of management experience.

All cells in the human body should contain 46 chromosomes, organised into 23 pairs. We inherit 23 chromosomes from our mother and 23 from our father. When an embryo has the correct number of chromosomes, it is called euploid.

An aneuploid embryo has too many or too few chromosomes, usually because of an error in the egg, and less commonly in the sperm. This error occurs before fertilisation. When fertilisation happens, every cell that develops from that embryo will carry the abnormal chromosome number.

A mosaic embryo is different. In this case, fertilisation occurs normally, and the embryo initially has the correct number of chromosomes. However, during one of the first cell divisions in the first few days of development, an error occurs in one cell. From that point on, the embryo develops two cell lines: one normal and one abnormal. This coexistence of healthy and abnormal cells defines mosaicism.

Mosaic embryos are identified using PGT-A, most commonly through next-generation sequencing (NGS). The reported rate of mosaicism depends heavily on the laboratory, the biopsy technique, and how strict the analytical thresholds are.

If the testing criteria are too strict, many embryos that would develop normally may be classified as mosaic. This explains why some studies report mosaic rates as low as 2%, while others report rates as high as 20–25%. The true rate is likely somewhere in between, closer to around 5%.

Traditionally, classification is based on the proportion of abnormal cells detected in the biopsy. Low-level mosaicism is generally defined as less than 40–50% abnormal cells, while high-level mosaicism is defined as more than 50%.

These thresholds are technical. Typically, five cells are biopsied, so results appear in increments of 20%. This is why the cut-off values exist. However, current evidence shows that low-level mosaics behave the same as euploid embryos, and many laboratories no longer report them separately.

There should no longer be any debate. Low-level mosaic embryos have the same implantation rates, miscarriage rates, and outcomes as euploid embryos. They should be transferred, and in many laboratories, they are now reported as euploid.

Yes. High-level mosaic embryos implant less frequently and have higher miscarriage rates compared to euploid embryos, but they still offer a meaningful chance of success. The live birth rate is approximately 25–28%.

Importantly, babies born from high-level mosaic embryos do not show a significantly increased risk of congenital abnormalities compared to euploid embryos. In my opinion, high-level mosaic embryos should be transferred.

True aneuploid embryos, where all cells are abnormal, should not be transferred. Mosaic embryos are different because they contain both healthy and abnormal cell lines.

Research suggests that abnormal cells tend to die off rather than persist. Studies analysing entire embryos donated for research have shown that mosaicism is present in the inner cell mass—the part that forms the baby—only around 15–25% of the time. This supports the concept of self-correction, particularly when mosaicism is limited.

Yes. Segmental mosaic embryos should be prioritised before whole-chromosome mosaics. Segmental mosaics generally have higher implantation rates and lower miscarriage rates.

If multiple high-level mosaics are available, embryos with fewer affected chromosomes are usually prioritised. However, current studies are too small to definitively rank specific chromosomes by outcome.

Patients should always have a consultation with a geneticist to understand the condition identified by PGT-A. I then explain the actual chances of implantation, miscarriage, and live birth.

For high-level mosaics, there is a slightly increased risk of miscarriage and a marginally increased risk of fetal abnormality. However, this increase is small. Patients must also understand that if they are unwilling to transfer mosaic embryos, performing PGT-A may actually reduce their overall chance of having a baby.

If mosaic embryos are excluded from transfer, PGT-A can eliminate embryos that still have a significant chance of producing a healthy baby. Some embryos classified as mosaic or even segmental aneuploid have resulted in healthy live births.

Using PGT-A without being willing to transfer mosaic embryos can therefore reduce the chance of pregnancy.

No. Unlike aneuploidy, mosaicism does not correlate with maternal or paternal age. Aneuploidy originates from errors in the egg or sperm, whereas mosaicism occurs later during embryo division and appears to be a random event.

Yes. Standard prenatal screening, such as NIPT and first-trimester assessments, is recommended for reassurance, as it would be for any pregnancy following PGT-A.

Transfer them. For patients who only have mosaic embryos, this understanding can mean the difference between having no chance of a baby and having a real chance. With appropriate counselling and follow-up screening, outcomes are overwhelmingly positive.

Do not isolate yourself. Surround yourself with people who offer support, not advice. Much of the advice patients receive is not evidence-based and often creates unnecessary guilt. The most important message is that mosaic embryos can and do lead to healthy babies, and patients deserve clear, honest information to make informed decisions.