Mosaic Embryos — High vs. Low Mosaicism. What Does It Mean for Your IVF Outcome?

Dr Ibrahim El Khatib, IVF Laboratory Director at ART Fertility Clinics, UAE: I am an embryologist with over 15 years of experience in IVF laboratories. He currently works across ART Fertility Clinics in Abu Dhabi and Dubai and is actively involved in patient-centred IVF laboratory practice.

From a patient perspective, it is important to understand that PGT-A does not change the quality or destiny of an embryo. It is a selection tool. If a patient has five or ten embryos, PGT-A helps narrow that group to those with the highest chance of success, for example, choosing one out of three rather than one out of ten.

PGT-A does not necessarily improve cumulative pregnancy rates per cycle, but it can shorten the time to pregnancy and, importantly, reduce the risk of miscarriage by avoiding the transfer of chromosomally abnormal embryos. Miscarriage is often discussed lightly, but its psychological impact can be comparable to losing a family member. There is also evidence that miscarriage can affect endometrial receptivity and cause further complications.

From a laboratory perspective, PGT-A has evolved significantly. Initially, testing was done on polar bodies, then on cleavage-stage embryos on day 3, where a single cell was removed. Today, testing is performed at the blastocyst stage by biopsying a few cells from the trophectoderm. These cells are sent to the genetic laboratory for analysis, and the results are used to guide transfer decisions.

Dr Antonio Capalbo, Chief Scientific Officer at Juno Genetics: I am a Chief Scientific Officer at Juno Genetics and an Assistant Professor of Medical Genetics in Italy. I have more than 15 years of experience in reproductive genetics and PGT and have led extensive research on mosaic embryos and embryo selection strategies.

Once the biopsy arrives, we lyse the cells to extract DNA and perform genetic analysis, primarily using next-generation sequencing. I fully agree that PGT-A is better described as a deselection tool. Its primary goal is to identify embryos with almost zero reproductive potential, particularly those with uniform, meiotically derived aneuploidies that affect all cells of the embryo.

These chromosomal abnormalities increase with maternal age and explain a large proportion of implantation failure and infertility. PGT-A is highly accurate at detecting these abnormalities and allows clinicians to deselect embryos that are extremely unlikely to result in a live birth.

With advances in technology, we can now also detect triploidy, haploidy, contamination, and confirm sample identity through embryo fingerprinting. The key priority in any PGT laboratory should be delivering a highly reliable test for detecting uniform aneuploidies.

Dr Ibrahim El Khatib, IVF Laboratory Director at ART Fertility Clinics, UAE: We traditionally assumed that all cells in a blastocyst had the same chromosomal makeup. With next-generation sequencing, we can now detect cases where some cells are normal, and others are aneuploid. For example, if a biopsy contains four cells and two are normal while two have an extra chromosome, this would be reported as mosaicism.

Initially, PGT-A results were simply euploid or aneuploid. Mosaicism introduced a third category, which created uncertainty. Over time, outcomes from mosaic embryo transfers showed that many resulted in healthy pregnancies.

Further distinctions were then introduced, such as low versus high mosaicism, whole chromosome versus segmental mosaicism, and single versus multiple chromosome involvement. This made interpretation more complex and highlighted that decisions depend heavily on laboratory validation and technology.

Natural mosaicism rates in pregnancies are approximately 1–2%. However, some laboratories report mosaicism rates as high as 20–30% in IVF embryos, which is inconsistent with natural data. This suggests technical and analytical factors play a significant role.

One of the most important studies was the non-selection study led by Dr Capalbo, where mosaic embryos were reported as euploid and transferred based on morphology alone. After transfer, outcomes were analysed, and no differences were found in implantation, miscarriage, or obstetric outcomes compared with euploid embryos. This showed that many mosaic embryos behave clinically like euploid embryos.

Dr Antonio Capalbo, Chief Scientific Officer at Juno Genetics: I am not against providing information, but I am against providing misleading or meaningless information. True mosaicism involving the inner cell mass is extremely rare in human pregnancies. We transferred embryos for decades before PGT without seeing increased mosaic-related complications.

The problem lies in technical limitations. There is currently no PGT method that can reliably predict true embryonic mosaicism. What we detect is an intermediate copy number signal in a biopsy, which does not necessarily reflect the embryo itself.

Different laboratories use different thresholds, leading to wide variation in reported mosaic rates. This inconsistency is not biological but technical. Evidence from large, double-blinded, prospective studies shows that embryos with these intermediate signals perform the same as euploid embryos, regardless of whether they are labelled low or high mosaic.

The lesson for the field is that new findings must be rigorously validated before being incorporated into clinical decision-making.

Dr Antonio Capalbo, Chief Scientific Officer at Juno Genetics: In most cases, mosaicism detected in a biopsy is not confirmed in the embryo or resulting pregnancy. Whether this reflects self-correction or a false-positive result is difficult to determine. What is clear is that in more than 99% of cases, mosaicism reported at biopsy is not observed later. Interpretation depends heavily on laboratory validation and methodology.

Dr Ibrahim El Khatib, IVF Laboratory Director at ART Fertility Clinics, UAE: I do not believe DNA fragmentation alone causes mosaicism. Technical issues may lead to chaotic or low-quality genetic results, but true mosaicism affecting a specific chromosome in a subset of cells is unlikely to be induced by routine IVF laboratory practices.

The number of cells biopsied, cell quality, and laboratory coordination are critical. Genetic laboratories rely on the quality of the biopsy material provided. Poor input leads to unreliable output. This is why close collaboration between IVF and genetics laboratories is essential.

True biological mosaicism exists, but it is rare and occurs at rates similar to natural conception. The risks associated with transferring a true mosaic embryo exist but are very low.

Dr Antonio Capalbo, Chief Scientific Officer at Juno Genetics: Based on all available evidence, mosaic patterns detected in trophectoderm biopsies do not affect the risk of having a mosaic pregnancy, nor do they affect obstetric or fetal outcomes. This finding is consistent across multiple studies. There is no evidence to suggest an increased risk to the health of the child later in life when embryos with these mosaic patterns are transferred.

Dr Ibrahim El Khatib, IVF Laboratory Director at ART Fertility Clinics, UAE: We perform laser-assisted hatching at the time of biopsy, not in advance. This can be on day five, six, or seven, once the embryo is well expanded and the trophectoderm contains an adequate number of cells. If the embryo is not hatching spontaneously, we perform laser hatching at that moment. We do not perform hatching on day 3 or day 4.

Dr Ibrahim El Khatib, IVF Laboratory Director at ART Fertility Clinics, UAE: We previously performed hatching on day three or day four, but we stopped because it created additional challenges. We observed a higher rate of fully hatched embryos, which are more sensitive and more difficult to manipulate. In many cases, embryos hatched from the inner cell mass, requiring additional intervention.

We also believe that embryo collapse before freezing improves cryopreservation outcomes. Early hatching did not support this. By avoiding early hatching, embryo grading became more objective, expansion was easier to assess, and biopsy placement could be better controlled. We consistently position the inner cell mass away from the biopsy site. The embryo remains within the zona pellucida and is fully collapsed at freezing. Overall, this approach improved consistency and control.

Some laboratories continue early hatching to increase the number of day 5 embryos, but this may mask slower embryo development.

Dr Antonio Capalbo, Chief Scientific Officer at Juno Genetics: We follow the same protocol. Opening the zona on day five at the blastocyst stage has become widely adopted, including across many clinics in Italy. This approach allows biopsy at the optimal time, when the blastocyst is fully expanded, and enables precise positioning away from the inner cell mass.

This method avoids fully hatched embryos and offers several technical advantages. Whether biopsy is performed using a flicking technique or direct blastocyst biopsy, outcomes are very good when procedures are done correctly. There is no evidence that the biopsy itself negatively affects embryo potential when performed by experienced professionals.

PGT is a complex procedure that requires expertise on both the IVF and genetics sides. Success depends on close collaboration between both teams.

Dr Antonio Capalbo, Chief Scientific Officer at Juno Genetics: The answer depends on the technology used and the level of validation behind it. Safety cannot be discussed without considering how mosaicism is detected and validated.

In some platforms, follow-up testing with alternative technologies has shown that certain profiles reported as mosaic actually represent technical variation within the aneuploid spectrum, often of meiotic origin. In these cases, what appears to be mosaicism may reflect technical noise rather than true biology.

In our recent blinded study at Juno Genetics, involving more than 15,000 single embryo transfers, we found that neither chromosome type nor the reported level of mosaicism provided any predictive value for implantation or embryo development. For this reason, we do not distinguish embryos based on mosaic level and avoid reporting findings that generate confusion without clinical utility.

Ultimately, the interpretation of mosaicism depends entirely on the technology used and the robustness of its validation.

Dr Ibrahim El Khatib, IVF Laboratory Director at ART Fertility Clinics, UAE: From a patient’s perspective, laboratory choice is often outside their control. A practical step patients can take is to ask about the laboratory’s mosaicism rate. If a laboratory reports mosaicism in 3% of embryos, decision-making differs greatly from a laboratory reporting 20%.

Some recent studies have shown that a proportion of embryos reported as mosaic are actually whole-chromosome or meiotic in origin. If no other embryos are available, factors such as which chromosome is affected become relevant. Outcomes differ depending on whether the chromosome involved is compatible with life.

If an embryo represents a full aneuploidy, it is unlikely to result in pregnancy. These nuances highlight the importance of understanding laboratory reporting practices.

Dr Antonio Capalbo, Chief Scientific Officer at Juno Genetics: Scientific societies must establish strict frameworks for validating genetic tests before findings are reported clinically. In clinical genetics, reporting requires analytical validity, clinical validity, clinical utility, and cost-effectiveness. PGT should follow the same principles.

Currently, many laboratories validate mosaic thresholds using cell lines, which are not comparable to embryo biopsies. Biopsies vary in cell number, quality, shipment conditions, and reagent batches. What is learned in validation settings often cannot be transferred to real clinical practice.

For uniform aneuploidies, validation is strong. For mosaic abnormalities, it is not. Until this changes, patients and clinicians should request validation data from providers and ask what predictive value these findings truly have.