Mosaicism in IVF: No Impact on Live Birth Rates – Study

The promise of preimplantation genetic testing for aneuploidy (PGT-A) has always been to improve IVF success rates by identifying embryos with the best chance of resulting in a healthy pregnancy. However, a large, rigorously designed study published in April 2026 in the American Journal of Obstetrics and Gynecology challenges a key aspect of that promise: the clinical value of reporting “mosaicism” in embryos. The research indicates that identifying embryos with a mix of normal and abnormal cells – mosaic embryos – through PGT-A does not reliably predict live birth rates.

What Does Mosaicism Mean for IVF?

PGT-A involves analyzing a compact number of cells from an embryo to check for an abnormal number of chromosomes, known as aneuploidy. Aneuploidy can lead to failed implantation, miscarriage, or genetic disorders. Mosaicism occurs when some cells within an embryo have a normal chromosome count, while others do not. Until recently, these mosaic results were often viewed as a sign of potential risk, leading some clinics to discard these embryos or transfer only those with a higher percentage of normal cells. This latest study casts doubt on whether that practice is justified.

The study, conducted across multiple US and European fertility clinics, involved nearly 15,000 single embryo transfers. Researchers found that reporting the presence of mosaicism – based on the proportion of cells with chromosomal abnormalities – did not improve the prediction of whether a transfer would result in a live birth. In other words, knowing an embryo was mosaic didn’t help clinicians determine its viability any better than if they hadn’t known. This finding is significant because it suggests that the added cost and complexity of analyzing for mosaicism may not be providing meaningful clinical benefit.

Study Design and Limitations

The research employed a double-blinded, non-selection study design. This means that clinicians were unaware of the mosaicism results when deciding which embryos to transfer, and the results were independently validated across different clinics and continents. This robust methodology strengthens the findings. However, it’s important to note the study’s limitations. The study focused on single embryo transfers, and the results may not be generalizable to cases where multiple embryos are transferred. The study assessed live birth rates, but did not examine other outcomes, such as miscarriage rates or the health of children born from mosaic embryos. The authors acknowledge that further research is needed to fully understand the long-term implications of transferring mosaic embryos. You can find more details about the study here.

What Does This Mean for Patients Undergoing IVF?

For individuals and couples undergoing IVF, this research suggests that the presence of mosaicism should not be the sole determining factor in embryo selection. The decision to transfer a mosaic embryo should be made in consultation with a qualified reproductive endocrinologist, taking into account individual patient factors, such as age, medical history, and previous IVF attempts. It’s crucial to understand that a mosaic result is not a definitive diagnosis of a failed pregnancy. Many mosaic embryos can still result in healthy babies.

The findings also highlight the evolving understanding of mosaicism in embryos. As Dr. Ruben Alvero discusses in this article, mosaicism has fundamentally changed how embryos are evaluated during PGT. The technology to detect mosaicism has advanced rapidly with the adoption of next-generation sequencing, but interpreting the clinical significance of these findings has lagged behind. This study contributes to a growing body of evidence suggesting that the clinical significance of mosaicism may be less clear-cut than previously thought.

The Role of PGT-A and Mosaicism Reporting

PGT-A remains a valuable tool for identifying embryos with high levels of aneuploidy, which are less likely to implant or result in a healthy pregnancy. However, the study suggests that focusing solely on intermediate levels of mosaicism may not be the most effective way to improve IVF outcomes. The authors argue that reporting mosaicism based on intermediate copy number deviations lacks clinical predictive value. This doesn’t mean PGT-A is ineffective, but rather that the interpretation of results needs refinement.

What’s Next for Mosaicism Research and Clinical Practice?

The findings of this study are likely to prompt a reevaluation of how mosaicism is reported and interpreted in clinical practice. Professional societies, such as the American Society for Reproductive Medicine (ASRM), may issue updated guidelines on the clinical management of embryos with mosaic results. The ASRM has previously addressed mosaicism in PGT, and this new research will likely inform future revisions to their recommendations. You can find more information about clinical management of mosaic results here.

Further research is needed to better understand the factors that influence the developmental potential of mosaic embryos. Studies are underway to investigate the relationship between the level of mosaicism, the tissue distribution of abnormal cells, and the likelihood of live birth. Researchers are also exploring the use of more sophisticated techniques to assess embryo quality, such as artificial intelligence-based image analysis. The goal is to develop more accurate and reliable methods for predicting which embryos have the best chance of resulting in a healthy pregnancy, while minimizing the burden and cost of PGT-A.

For now, patients considering IVF should engage in open and honest conversations with their fertility specialists about the potential benefits and limitations of PGT-A and the implications of mosaicism results. A personalized approach, based on individual circumstances and the latest scientific evidence, is essential for making informed decisions about embryo selection and maximizing the chances of a successful pregnancy.