If you've gone through PGT-A testing as part of your IVF cycle, you may have received a result you weren't expecting: one or more of your embryos came back as "mosaic." For many intended parents, this word can feel confusing—or even alarming. Does it mean the embryo is abnormal? Should it be discarded? Can it still lead to a healthy pregnancy?
In short: a mosaic embryo is not automatically non-viable. Current ASRM guidance supports individualized consideration of mosaic embryo transfer, especially when no euploid embryos are available. Low-level and segmental mosaics may have outcomes closer to euploid embryos than previously thought, while high-level or complex mosaics warrant greater caution. Genetic counseling and prenatal diagnostic follow-up remain essential in all cases.
In its 2023 revised Committee Opinion and 2024 PGT-A Committee Opinion, the American Society for Reproductive Medicine (ASRM) provided updated, evidence-based guidance that paints a far more nuanced—and often reassuring—picture than what many patients have been told in the past.
In this article, we'll break down what mosaic embryos actually are, what the latest guidelines say, and what all of this means for intended parents navigating a surrogacy journey.
Key Takeaways
- A "mosaic" PGT-A result does not necessarily mean your embryo is abnormal. It indicates an intermediate chromosomal reading that could reflect true mosaicism, but may also be caused by technical factors in the testing process itself. To learn more about how and why PGT-A can produce inaccurate results, see our in-depth guide on PGT-A false positives.
- Mosaic embryos should not be automatically discarded. Both the ASRM and the Preimplantation Genetic Diagnosis International Society (PGDIS) now recognize that mosaic embryos can—and do—result in healthy live births.
- Mosaic embryo transfers show meaningful success rates. Published data show live birth rates of approximately 30–50% for mosaic embryo transfers, compared to roughly 50–60% for euploid transfers. Lower than euploid, but far from zero.
- Once a mosaic embryo successfully implants, outcomes are reassuring. Miscarriage rates after implantation are comparable between mosaic and euploid transfers. Most reported live births after mosaic embryo transfer have had reassuring neonatal outcomes, though long-term follow-up data remain limited.
- PGT-A only samples the trophectoderm—cells that become the placenta, not the baby. A mosaic reading from this outer cell layer may not reflect the chromosomal status of the inner cell mass, which is what actually develops into the fetus. This is one of the most important biological reasons why mosaic embryos can still produce healthy pregnancies.
- The same embryo can receive different results at different laboratories. There is no universal standard for reporting mosaicism. What one lab calls "mosaic," another might call "euploid" or "aneuploid."
- Embryos may have self-correction mechanisms. Growing evidence suggests that embryos can eliminate chromosomally abnormal cells through natural biological processes during early development—though this does not guarantee correction in any individual embryo.
- Genetic counseling is strongly recommended before making any decision about transferring a mosaic embryo, especially in the surrogacy context where transfer opportunities are limited.
What Is a Mosaic Embryo?
In medical genetics, "mosaicism" refers to the presence of two or more genetically distinct cell populations within a single organism. In the context of IVF, a mosaic embryo is one where PGT-A testing detects a mixture of chromosomally normal (euploid) and chromosomally abnormal (aneuploid) cells in the trophectoderm biopsy—the small cluster of cells sampled from the outer layer of the blastocyst.
Here's a helpful way to think about it: imagine a bag of 100 marbles. If all 100 are blue, that's a euploid embryo. If all 100 are red, that's an aneuploid embryo. But if 70 are blue and 30 are red, that's a mosaic—a mix.
It's important to understand that PGT-A doesn't actually count individual cells one by one. Instead, it uses next-generation sequencing (NGS) technology to analyze the collective DNA from a group of cells and infers the chromosomal makeup from the resulting pattern. What shows up as an "intermediate" reading could represent true biological mosaicism—or it could be an artifact of the testing process itself, caused by factors such as DNA amplification bias, statistical noise, contamination, or variations in biopsy technique. These technical limitations are closely related to the broader issue of PGT-A false positives—cases where an embryo is labeled abnormal or mosaic when it may, in fact, be chromosomally normal.
How Common Are Mosaic Results?
The reported rate of mosaic results varies significantly—from as low as 2% to over 20% of all PGT-A tested embryos. This wide range reflects differences in laboratory technologies, bioinformatics algorithms, and reporting thresholds rather than true biological differences in the embryos.
The ASRM specifically highlights this point: the same trophectoderm biopsy could be classified as "high-level mosaic" at one laboratory and either "aneuploid" or "euploid" at another. For intended parents, this means that a mosaic diagnosis should always be interpreted in the context of the specific laboratory's reporting criteria—and ideally discussed with a genetic counselor who understands these nuances.
What Do the Updated ASRM Guidelines Say?
Mosaic Embryos Can Lead to Healthy Babies
This is perhaps the most important message from the updated guidelines. Since the first documented live births from known mosaic embryos in 2015, hundreds of healthy babies have been born from such transfers. The ASRM and PGDIS both now formally recognize that mosaic embryo transfers can result in healthy live births and should not be automatically discarded. Most reported neonatal outcomes have been reassuring, though providers and PGT-A laboratories are encouraged to continue tracking and publishing long-term follow-up data.
Outcomes Are Lower Than Euploid, But Meaningful
Mosaic embryo transfers are associated with somewhat lower implantation and live birth rates compared to euploid transfers. A 2025 systematic review and meta-analysis confirmed this pattern—but crucially found that once implantation occurs, miscarriage rates were comparable between mosaic and euploid groups. This suggests that implantation is the main hurdle, not ongoing pregnancy viability.
Across published studies, live birth rates for mosaic embryo transfers range from approximately 27% to 51%, depending on study design, patient population, level and type of mosaicism, and embryo selection criteria. For comparison, euploid embryo transfers typically yield live birth rates of 50–60%.
Low-Level Mosaics: Closer to Euploid Than You Might Think
An important real-world clinical nuance: not all mosaic results carry the same implications. Low-level mosaicism (typically 20–40% aneuploid DNA) has increasingly been associated with clinical outcomes approaching those of euploid embryos. Many fertility centers now prioritize low-level mosaic embryos for transfer with a high degree of clinical confidence, particularly when the mosaicism is segmental rather than whole-chromosome. A prospective non-selection trial by Capalbo et al. (2021) found comparable rates of live birth and miscarriage for euploid embryos and embryos with low-to-medium level mosaicism.
That said, high-level mosaicism (40–80%) warrants more cautious evaluation and deeper genetic counseling, as the data are less consistent and the clinical picture is more uncertain.
How Mosaic Embryos Are Prioritized for Transfer
When no euploid embryos are available, both the ASRM and PGDIS have published guidance on ranking mosaic embryos. The following table summarizes the general prioritization framework, though the ASRM notes that the influence of these factors on outcomes has been inconsistent across studies, and decisions should always be individualized:
Factor | More Favorable | Less Favorable |
|---|---|---|
Level of mosaicism | Low-level (20–40%) | High-level (40–80%) |
Type of abnormality | Segmental (part of a chromosome) | Whole-chromosome |
Monosomy vs. trisomy | Monosomy (except 45,X) — non-viable cells tend to be eliminated naturally | Trisomy |
Specific chromosomes involved (trisomy) | Chromosomes not associated with viable syndromes (e.g., 1, 3, 4, 5, 6, 8, 9, 10, 11, 12, 17, 19, 20, 22) | Chromosomes 13, 18, 21 (associated with viable trisomy syndromes — lowest priority) |
Additional caution | — | Chromosomes 14, 15 (uniparental disomy risk); Chromosomes 2, 7, 16 (intrauterine growth restriction risk) |
This framework draws on PGDIS position statements and ASRM 2023 guidance, though no universally validated scoring system currently exists. A genetic counselor can help interpret your specific result in the context of these considerations.
Genetic Counseling Is Essential
The ASRM strongly recommends that any patient considering the transfer of a mosaic embryo consult with a board-certified genetic counselor specializing in PGT-A interpretation. This recommendation extends through pregnancy—prenatal diagnostic testing should be offered to all patients who conceive after a mosaic embryo transfer, with amniocentesis generally preferred over CVS because CVS samples placental tissue—the same lineage PGT-A already tested.
Why Can Some Mosaic Embryos Still Result in Healthy Pregnancies?
One of the most fascinating areas of emerging research is embryonic self-correction. These mechanisms are biologically plausible and supported by experimental data, but they do not guarantee that any individual mosaic embryo will self-correct. With that important caveat, scientists have identified three main models by which embryos may manage chromosomally abnormal cells:
The Clonal Clearance Model: Aneuploid cells undergo programmed cell death (apoptosis) at higher rates than euploid cells. Research shows this process is especially active in the inner cell mass (ICM)—the cells that become the baby—with apoptosis rates of approximately 41% in aneuploid ICM cells versus 20% in euploid ICM cells in animal models.
The Exclusion Model: Abnormal cells may be physically expelled from the embryo as cell debris. A 2020 human embryo study provided direct evidence: of nine euploid blastocysts examined, five had corresponding expelled cell debris that carried aneuploid DNA.
The Selective Distribution Model: Aneuploid cells may be preferentially allocated to the trophectoderm (placenta) rather than the ICM (fetus), effectively quarantining abnormal cells away from the developing baby.
These mechanisms may help explain why many embryos labeled "mosaic" from a trophectoderm biopsy can still produce healthy pregnancies—and why some embryos classified as mosaic on initial biopsy show euploid results when re-biopsied. However, current science cannot predict which specific mosaic embryos will benefit from these processes, which is why individualized genetic counseling remains essential.
Why This Matters Especially in Surrogacy
For intended parents pursuing surrogacy, the question of whether to transfer a mosaic embryo carries particular weight. In standard surrogacy agreements, there is typically a limited number of embryo transfers permitted within a defined time window. Each transfer cycle involves significant coordination, medical preparation for the gestational carrier, and financial investment.
This means that every transfer attempt matters more. For intended parents pursuing surrogacy, embryo strategy is not only a lab decision—it is also a timeline, cost, and legal-planning decision. A viable mosaic embryo that is unnecessarily discarded—or an extra IVF cycle pursued when a transferable mosaic was available—can have disproportionate consequences.
A 2024 decision-analytic model (Khorshid et al., J Assist Reprod Genet) directly examined this trade-off. For patients under 43, an additional IVF cycle with PGT-A yielded a higher live birth rate than transferring a mosaic embryo to a gestational carrier—but at a substantial average additional cost of approximately $16,600. For patients over 42, mosaic embryo transfer actually produced a higher live birth rate while costing approximately $9,600 less. The authors concluded that mosaic embryo transfers should be seriously considered when additional IVF cycles are not feasible.
In the surrogacy context—where the intended mother's age may make additional egg retrievals difficult or impossible—understanding these trade-offs becomes essential for informed decision-making.
What Questions Should You Ask Your Clinic?
If you've received a mosaic PGT-A result, here are some informed questions to bring to your medical team:
About the result: What are the lab's specific thresholds for calling an embryo mosaic? What is the level of mosaicism (low vs. high)? Is it segmental or whole-chromosome? Which chromosome(s) are involved?
About your options: Do I have euploid embryos available? Would re-biopsy provide useful additional information? What does my RE recommend given my specific situation?
About next steps: Can I be referred to a genetic counselor who specializes in PGT-A? If pregnancy occurs, what prenatal diagnostic testing will be offered?
The Bottom Line
A mosaic PGT-A result is not a diagnosis of abnormality—it's a finding that requires careful interpretation. The latest evidence, reflected in the 2023 ASRM Committee Opinion and 2024 PGT-A guidelines, makes clear that mosaic embryos deserve thoughtful consideration, not automatic discard. With appropriate genetic counseling, individualized risk assessment, and proper prenatal follow-up, many mosaic embryos go on to produce healthy pregnancies and healthy babies.
For intended parents on a surrogacy journey, understanding mosaic embryos empowers you to have more informed conversations with your medical team and make decisions that reflect both the best available science and your personal circumstances.
If you have questions about embryo screening results and how they may affect your surrogacy plan, our team is here to help. Contact us or fill out our Parent Application Form to connect with a surrogacy specialist who can guide you through the next steps.
Frequently Asked Questions
Q: Does a mosaic PGT-A result mean my embryo is definitely abnormal?
A: No. A mosaic result indicates an intermediate chromosomal reading in the trophectoderm biopsy. This could reflect true mosaicism, but it could also be caused by technical factors such as DNA amplification bias or statistical noise. The ASRM notes that a mosaic result can correspond to an embryo that is actually euploid, aneuploid, or truly mosaic. For a deeper look at how testing inaccuracies occur, see our article on PGT-A false positives.
Q: Can mosaic embryos lead to healthy babies?
A: Yes. Both the ASRM and PGDIS recognize that mosaic embryo transfers have resulted in hundreds of healthy live births since 2015. Published data show live birth rates ranging from approximately 27% to 51%. Most reported neonatal outcomes have been reassuring, though long-term follow-up data are still being collected.
Q: Should I discard my mosaic embryos?
A: Current guidelines strongly advise against automatically discarding mosaic embryos. They should be considered as potential candidates for transfer—particularly when no euploid embryos are available. A genetic counselor can help you assess the relative risks based on your specific embryo characteristics.
Q: What's the difference between low-level and high-level mosaicism?
A: Low-level mosaicism typically refers to 20–40% aneuploid cells, while high-level refers to 40–80%. Clinical data increasingly suggest that low-level mosaic embryos perform close to euploid embryos in terms of pregnancy outcomes, and many fertility centers treat them with growing clinical confidence. High-level mosaics warrant more cautious evaluation and in-depth genetic counseling.
Q: Is mosaic embryo transfer considered an acceptable option in surrogacy?
A: Yes. Mosaic embryo transfer is considered an acceptable clinical option when performed with appropriate genetic counseling, informed consent from all parties, individualized review of the specific mosaic result, and a plan for prenatal diagnostic follow-up. The clinical considerations for mosaic embryo transfer apply regardless of whether the pregnancy is carried by the intended parent or a gestational carrier. Prenatal diagnostic testing—typically amniocentesis—should be offered for any pregnancy resulting from a mosaic embryo transfer.
Q: Why is amniocentesis generally preferred over NIPT for confirming mosaic pregnancies?
A: NIPT (non-invasive prenatal testing) analyzes cell-free DNA circulating in the mother's blood—but this DNA originates from the placenta, which is the same tissue lineage that PGT-A already tested. Using NIPT would essentially be re-testing the same cell source. Amniocentesis, by contrast, directly samples fetal cells from the amniotic fluid, providing a definitive assessment of the baby's own chromosomal status. This is why amniocentesis is generally the preferred confirmatory test when prenatal diagnosis is pursued after mosaic embryo transfer.
Q: Why can some mosaic embryos still result in healthy pregnancies?
A: Research has identified several biologically plausible mechanisms. Aneuploid cells may undergo programmed cell death (apoptosis) at higher rates than normal cells; abnormal cells may be physically expelled from the embryo; or abnormal cells may be preferentially directed to the placental tissue rather than the fetal tissue. These processes are supported by experimental data but do not guarantee correction in any individual embryo—which is why genetic counseling and prenatal follow-up remain important.
Q: If the test only samples the outer cells, how do we know what's happening inside?
A: This is a key limitation of PGT-A. The biopsy samples trophectoderm cells (future placenta), not the inner cell mass (future baby). Studies have found that results do not always match between these two cell populations. Some research indicates that nearly half of embryos classified as mosaic on initial biopsy showed euploid results on re-biopsy, underscoring why a mosaic PGT-A reading should not be taken as a definitive diagnosis.
References
- Practice Committees of the ASRM and the Genetic Counseling Professional Group. Clinical management of mosaic results from preimplantation genetic testing for aneuploidy (PGT-A) of blastocysts: a committee opinion. Fertil Steril. 2023;120(5):973–82. PubMed
- Practice Committees of the ASRM and the Society for Assisted Reproductive Technology. The use of preimplantation genetic testing for aneuploidy: a committee opinion. Fertil Steril. 2024;122(3):421–434. PubMed
- Leigh D, Cram DS, Rechitsky S, et al. PGDIS position statement on the transfer of mosaic embryos 2021. Reprod Biomed Online. 2022;45(1):19–25. PubMed
- Capalbo A, Poli M, Rienzi L, et al. Mosaic human preimplantation embryos and their developmental potential in a prospective, non-selection clinical trial. Am J Hum Genet. 2021;108(12):2238–2247. PubMed
- Viotti M, Victor AR, Barnes FL, et al. Using outcome data from one thousand mosaic embryo transfers to formulate an embryo ranking system for clinical use. Fertil Steril. 2021;115(5):1212–1224. PubMed
- Khorshid A, Bavan B, Chung EH, Lathi RB. Mosaic embryo transfer versus additional IVF with PGT-A cycle: a decision model comparing live birth rate and cost. J Assist Reprod Genet. 2024;41(3):635–641. PubMed
- Dal Molin EA, Pires GN, Lazzari VM. Live birth and other reproductive outcomes of mosaic and euploid embryos: a systematic review and meta-analysis. J Assist Reprod Genet. 2025;42(8):2469–2481. PubMed
