Mosaic Embryo Transfer: Should You Do It in 2026

Your PGT-A report came back with one or more mosaic results. There is a percentage next to a chromosome name and your RE has scheduled a counseling call. You are reading because you want to walk into that call understanding what "low-level" and "high-level" mean, which chromosome actually matters, and what the data show about live births from a mosaic embryo transfer.

Five years ago I would have said no to mosaic transfer. The data has shifted, and now I tell patients the answer depends on the chromosome involved, the percentage reported, what else is in the freezer, and whether genetic counseling and prenatal diagnostic testing fit your plan. Genetic counseling is not optional for this decision. The Capalbo 2021 non-selection trial, the PGDIS 2021 position statement, and large registry data from Viotti and colleagues have all moved the conversation from "no, never" to "case by case with full counseling"¹²⁵.

This post walks through what mosaic means on a report, why it happens biologically, what the recent data show, the decision tree most reproductive endocrinologists (REs) are using now, and what to ask before you choose. The decision is yours and your team's. My goal is to make sure you carry the same information into the room that I would.

What "mosaic" means on a PGT-A report

The five-cell trophectoderm biopsy returned both normal and abnormal cells. The report describes the result with a percentage of cells abnormal and a chromosome name. The percentage is usually grouped into categories that vary by lab:

• Low-level mosaic: roughly 20 to 50 percent abnormal cells
• High-level mosaic: roughly 50 to 80 percent abnormal cells
• Some labs report a single number; others use these buckets

Two further distinctions matter.

A single-chromosome mosaic involves one chromosome. A complex mosaic involves multiple. Single-chromosome mosaics generally carry better prognosis than complex.

A whole-chromosome mosaic involves the entire chromosome being affected in the abnormal cells. A segmental mosaic involves only part of a chromosome. Segmental mosaics generally carry lower transfer risk than whole-chromosome mosaics, though the data are still maturing.

So when the report says "low-level mosaic 30 percent, chromosome 9," that means the lab estimates roughly 30 percent of the cells in the biopsy showed an abnormality on chromosome 9. It does not mean 30 percent of the entire embryo is abnormal: the biopsy sampled five cells out of the trophectoderm, and trophectoderm is not the inner cell mass.

The biology: why mosaicism happens

A mosaic embryo arises from a post-zygotic mitotic error. Fertilization produces a single zygote with normal chromosomes. As the zygote divides, one or more cell divisions go wrong, and a daughter cell ends up with extra or missing chromosomes. From that point forward, the embryo carries a mix of normal and abnormal cell lines.

What makes mosaic transfer biologically defensible, and this is the central insight from the last decade of research, is self-correction during development. Abnormal cells can be preferentially displaced into the trophectoderm (the placenta-forming layer) while the inner cell mass (the fetus-forming layer) remains chromosomally normal. Healthy live births from mosaic transfers, including in the original Greco et al. 2015 NEJM report and the Capalbo 2021 trial, are consistent with this displacement model¹³.

The sampling limit also matters. Five cells out of the trophectoderm cannot represent the whole embryo. A "30 percent mosaic" reading may be a sampling artifact in an otherwise normal embryo, or it may reflect a wider abnormality. The test cannot tell you which.

The recent data shift

The decade between 2015 and 2025 produced steady evidence that mosaic transfer is more viable than the original "discard all abnormal" approach assumed.

• Greco et al. 2015, NEJM: first published reports of healthy live births after intrauterine transfer of mosaic blastocysts that had been retested³.
• Munné et al. 2017, F&S: registry data showing mosaic transfers produce live births at lower but real rates compared with euploid transfers⁴.
• PGDIS 2019 and 2021 position statements: opened the door to mosaic transfer in carefully counseled patients, with chromosome-specific guidance⁵.
• Capalbo et al. 2021, American Journal of Human Genetics: a prospective non-selection clinical trial in which mosaic embryos were transferred alongside euploids and outcomes compared. Low-level mosaics produced live birth rates approaching those of euploids in that cohort; high-level mosaics produced lower but still meaningful rates¹.
• Viotti et al. 2021, F&S: large multicenter registry analysis of over one thousand mosaic embryo transfers, used to build an embryo ranking system that has informed clinical practice since².
• ESHRE Working Group on Chromosomal Mosaicism 2022: survey results and good-practice recommendations for managing mosaic results, including counseling pathways and reporting thresholds⁶.

This is a body of evidence, not a single study. The convergence is what shifted clinical practice. Most academic centers now offer mosaic transfer with counseling, and outcomes data continue to support a measured yes for the right cases.

Mosaic embryo transfer: the decision tree REs use

Here is the structure of the conversation you are about to have. Your clinic may use a slightly different framework, but the inputs are usually the same.

• Euploid embryos available plus a low-level mosaic: transfer the euploid first. Bank the mosaic for later if needed.
• Euploid available plus a high-level mosaic: transfer the euploid first. Mosaic decision deferred.
• No euploid available plus a low-level mosaic on a lower-risk chromosome: consider transfer with full genetic counseling and an agreed prenatal testing plan.
• No euploid available plus a high-level mosaic or complex mosaic: counsel carefully. Some patients still proceed with detailed prenatal testing planned. Some choose another retrieval first.
• No euploid plus only aneuploid embryos in the freezer: a mosaic, even high-level, is generally preferred over an aneuploid for transfer. Aneuploid embryos are rarely transferred.
• Complex mosaic or mosaic involving trisomy 13, 18, or 21: more cautious. Genetic counseling required. Some clinics will not transfer; others will with specific informed consent.

The single most important input is the chromosome involved. Not all chromosomes carry equal risk, and the conversation in the consult room turns on chromosome identity first.

Chromosome-specific considerations

A short tour of why chromosome matters.

• Trisomies 13, 18, 21: viable to term. Patau, Edwards, and Down syndromes respectively. Mosaic results involving these chromosomes carry the highest concern because a live birth with the condition is possible. Most clinics will require detailed prenatal testing planning before transferring.
• Trisomy 16: the most common embryonic aneuploidy. Almost always results in first-trimester miscarriage. A mosaic 16 carries a different kind of risk: not a child with a syndrome, but a higher likelihood of pregnancy loss with the psychological burden that brings.
• Most other autosomal trisomies: not viable beyond first trimester. Mosaic results involving them generally carry miscarriage risk rather than live-birth-with-condition risk.
• Sex chromosome mosaics (XXY, XYY, XXX, monosomy X): variable outcomes. Some sex chromosome conditions are mild enough to be diagnosed only incidentally; others have substantial implications. Genetic counseling is essential.
• Segmental mosaics: usually lower risk than whole-chromosome. The specific segment and gene content involved drive the risk profile.

This is one of the reasons "mosaic" alone is not a transfer decision. The same percentage and the same level (low or high) can mean very different things across chromosomes.

Outcome data: what to expect

These ranges are drawn from the Capalbo non-selection trial, the Viotti registry analysis, and ongoing clinic data¹².

• Live birth rate per transferred low-level mosaic: roughly 40 to 50 percent in pooled data, approaching euploid rates in some cohorts.
• Live birth rate per transferred high-level mosaic: roughly 25 to 35 percent.
• Live birth rate per euploid transfer (for context): roughly 50 to 60 percent in the same age bands.
• Miscarriage rate after positive beta hCG: similar to euploid for low-level mosaics; somewhat higher for high-level.
• Confirmed mosaicism in the resulting child: very rare. The vast majority of live births from mosaic transfers produce chromosomally normal children, consistent with the self-correction model.

The data are robust enough that I now frame mosaic transfer as a real option in most counseling sessions. The numbers are not parity with euploid for high-level mosaics, but they are far from zero.

Recommended pre-transfer steps for a mosaic transfer

If you and your team decide to proceed with mosaic transfer, expect the following.

1. Genetic counseling appointment: Required at most centers. The counselor will walk through the specific chromosome, the percentage, segmental versus whole, and what prenatal testing would look like during pregnancy. You will also discuss what you would do with the information if prenatal testing returned a confirmed abnormality. This conversation is hard. Have it before transfer, not during pregnancy.
2. Detailed informed consent: Many clinics use mosaic-specific consent forms documenting the chromosome, the level, and the counseling that occurred.
3. Plan for non-invasive prenatal testing (NIPT) in the first or early second trimester to screen for the chromosome involved. NIPT is a screening test, not diagnostic.
4. Plan for chorionic villus sampling (CVS) or amniocentesis if NIPT is concerning or if you want diagnostic certainty regardless of NIPT result. Some patients opt for early diagnostic testing rather than serial screening.
5. Standard frozen embryo transfer (FET) cycle: lining preparation with estrogen and progesterone, transfer of the selected embryo, beta hCG ten to twelve days later.

The decision tree extends past the transfer. Plan the pregnancy-side testing now, with the genetic counselor, so the early weeks are not a separate decision-making sprint.

What to ask your RE before mosaic transfer

A list to bring to the counseling consult.

• What is the chromosome involved? Is it whole-chromosome or segmental?
• What is the percentage of abnormal cells reported, and what is your clinic's threshold for transfer at this level?
• Do I have any euploid embryos available to transfer first?
• What is your clinic's live birth rate for mosaic transfers in this category over the past two to three years?
• Will you require a genetic counseling appointment before scheduling transfer?
• What is the prenatal testing plan if transfer is successful?
• What is your policy on re-biopsy for mosaic embryos, and would re-biopsy change the recommendation?

A clinic that can answer these in numbers, without hedging, without "in general," is doing the work. A clinic that cannot is a flag.

What this means for you

The mosaic embryo transfer decision sits at the intersection of biology, statistics, and ethics. The biology supports transfer in many cases. The statistics support transfer for low-level mosaics on lower-risk chromosomes. The ethics, meaning what you would do if prenatal testing confirmed an abnormality, belong to you and your partner and need to be settled before transfer, not after.

If you have a euploid embryo and a mosaic, the transfer order is usually clear: euploid first. If you have only a mosaic, the decision is harder and the right answer for one couple is not the right answer for another. Take the genetic counseling appointment. Bring questions. Take time. Embryos in the freezer can wait.

What's next

• If you have a euploid embryo in addition to the mosaic and are heading to FET: embryo-grading-explained for how grade interacts with ploidy in transfer order
• If you want the broader PGT-A results context: pgt-a-results-meaning
• If you need the full PGT testing background: pgt-a-m-sr-explained
• If a previous transfer failed and you are now weighing mosaic in a different cohort: failed-ivf-decoding-next

Sources

1. Capalbo A, Poli M, Rienzi L, et al. Mosaic human preimplantation embryos and their developmental potential in a prospective, non-selection clinical trial. American Journal of Human Genetics 2021;108(12):2238-2247. https://doi.org/10.1016/j.ajhg.2021.11.002
2. 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. Fertility and Sterility 2021;115(5):1212-1224. https://doi.org/10.1016/j.fertnstert.2020.11.041
3. Greco E, Minasi MG, Fiorentino F. Healthy babies after intrauterine transfer of mosaic aneuploid blastocysts. New England Journal of Medicine 2015;373(21):2089-2090. https://doi.org/10.1056/NEJMc1500421
4. Munné S, Blazek J, Large M, et al. Detailed investigation into the cytogenetic constitution and pregnancy outcome of replacing mosaic blastocysts detected with the use of high-resolution next-generation sequencing. Fertility and Sterility 2017;108(1):62-71.e8. https://doi.org/10.1016/j.fertnstert.2017.05.002
5. Preimplantation Genetic Diagnosis International Society (PGDIS). PGDIS Position Statement on the Transfer of Mosaic Embryos 2021. https://www.pgdis.org/
6. ESHRE Working Group on Chromosomal Mosaicism, De Rycke M, Capalbo A, Coonen E, et al. ESHRE survey results and good practice recommendations on managing chromosomal mosaicism. Human Reproduction Open 2022;2022(4):hoac044. https://doi.org/10.1093/hropen/hoac044