A decade ago, PGT-A results were reported as binary: euploid (normal chromosomes) or aneuploid (abnormal chromosomes). Aneuploid embryos were typically not transferred. Then next-generation sequencing platforms became sensitive enough to detect embryos with mixed cell populations — some normal cells, some abnormal — and the concept of "mosaic" embryos entered clinical practice.
What started as a small subcategory has become a routine part of PGT-A reporting. Depending on the lab and platform, 5–20% of tested embryos are reported as mosaic. What to do with them is one of the most-debated questions in current IVF practice. This piece walks through the current classification system, what the evidence supports about transfer outcomes, and how the decision framework has evolved through 2026.
What "Mosaic" Actually Means
Mosaic embryos contain multiple cell lines with different chromosomal compositions. When PGT-A biopsies 5–10 cells from a blastocyst, some cells may show normal chromosomes and others may show abnormalities. The proportion of abnormal cells in the sample determines the mosaic classification.
Important caveat: PGT-A samples only trophectoderm cells (the outer layer that becomes placenta). The inner cell mass (which becomes the fetus) is not directly sampled. Mosaic results reflect the trophectoderm biopsy specifically — not necessarily the entire embryo. This is one reason mosaic results are more nuanced than binary results.
The Current Classification System
| Classification | % Abnormal Cells | Typical Recommendation |
|---|---|---|
| Euploid | <20% | Transfer as first-line option |
| Low-level mosaic | 20–40% | Transfer often acceptable in absence of euploid options; case-specific |
| High-level mosaic | 40–80% | More cautious approach; discussed case-by-case |
| Aneuploid | >80% | Generally not transferred |
Note: cutoff percentages vary somewhat between labs and reporting platforms. Some labs use 40% as the low/high mosaic threshold; others use 50%. Discuss your specific report with your REI to understand the exact classifications used.
What the Evidence Supports About Transferring Mosaic Embryos
Since 2016, when the first mosaic embryo transfers were systematically reported, outcome data has accumulated:
These numbers are approximate averages across multiple studies and vary substantially by patient age, specific chromosomes involved, and clinic. Individual outcomes may differ meaningfully.
| Claim | Evidence level | What's true |
|---|---|---|
| Low-level mosaic embryos can produce healthy live births | Strong | Well-documented across multiple case series |
| Mosaic embryo transfer outcomes vary by chromosome and mosaic percentage | Strong | Not all mosaic embryos are equivalent; specific chromosome matters |
| Prenatal testing after mosaic transfer typically shows normal karyotype | Moderate | Most confirmed pregnancies from mosaic transfers show normal fetal chromosomes; "self-correction" is one hypothesis |
| Mosaic embryos should be prioritized over euploid embryos | Not Supported | Euploid embryos remain first-line when available |
The Decision Framework Currently in Use
When you have euploid embryos available
Transfer euploid embryos first. Mosaic embryos are typically preserved for later consideration if euploid transfers don't achieve pregnancy.
When mosaic is your best option
Patients whose PGT-A results yield only mosaic embryos (no euploid) face a distinct decision:
- Low-level mosaic transfer has meaningful success rates and produces healthy pregnancies in most cases
- Specific chromosome involved matters — some chromosomes have better outcomes when involved in mosaicism than others
- Comprehensive prenatal testing (amniocentesis or CVS) is typically recommended if pregnancy occurs from a mosaic transfer
- Full genetic counseling before transfer is standard
When mosaic transfer is not typically recommended
- Trisomy 13, 18, or 21 mosaicism (chromosomes associated with viable but affected pregnancies)
- Very high-level mosaicism (>70–80% abnormal cells)
- Multiple chromosome involvement (complex mosaic)
Every patient considering a mosaic embryo transfer should have direct conversation with a genetic counselor before proceeding. The counselor reviews the specific chromosomes involved, the mosaic level, potential outcome scenarios, and prenatal testing options. This is standard responsible practice; a clinic that skips this step is skipping something important.
The "Self-Correction" Question
One of the more interesting observations from mosaic embryo transfer follow-up: most pregnancies that reach prenatal testing show normal fetal chromosomes, even from mosaic embryos. Several explanations are considered:
- Self-correction: abnormal cells are preferentially lost during development, leaving the fetus with predominantly normal cells
- Sampling bias: the trophectoderm biopsy overrepresented abnormal cells relative to the actual embryo composition
- Placental confinement: abnormal cells sequester in placental tissue while the fetus develops normally
These aren't fully understood mechanistically, but the clinical observation is fairly consistent: mosaic transfers that reach prenatal testing usually show normal fetal chromosomes. This influences the current willingness to transfer low-level mosaic embryos when they represent the best available option.
Colombia-Specific Considerations
Colombian IVF programs performing PGT-A generally follow international guidelines (PGDIS position statements and similar) for mosaic classification and transfer decisions. Genetic counseling access varies by program — larger academic programs typically have on-site geneticists; some smaller clinics work with telegenetic consultation networks.
For patients traveling to Colombia specifically for IVF with PGT-A, verifying the following at consultation is worth doing:
- Which lab performs the PGT-A analysis and what classification thresholds they use
- Whether genetic counseling is included in the treatment package
- The clinic's protocol for mosaic embryo transfer decisions
- Prenatal testing recommendations if a mosaic transfer results in pregnancy
Colombia's #1 Western Hemisphere / #22 global WHO healthcare ranking (per the 2000 World Health Report) is reflected in the broader medical infrastructure, but specific genetic testing quality depends on the specific lab and clinic. Verifying these details is part of due diligence.
Frequently Asked Questions
Can a mosaic embryo produce a healthy pregnancy?
Yes. Low-level mosaic transfers have well-documented healthy live births, and follow-up testing typically shows normal fetal chromosomes. The success rate is lower than euploid transfers but meaningfully high in absolute terms.
Should I transfer a mosaic embryo if I also have euploid options?
Generally, no. Euploid embryos are transferred first when available. Mosaic embryos are preserved and considered if euploid transfers don't result in pregnancy.
What if all my embryos come back mosaic?
This is when mosaic transfer becomes a real option. Discuss specifically with your REI and a genetic counselor, considering which chromosomes are involved and the mosaic level. Low-level mosaic transfer can produce successful pregnancies.
What prenatal testing is recommended after a mosaic transfer?
Comprehensive prenatal testing, typically including either amniocentesis or CVS, is standard. Non-invasive prenatal testing (NIPT) can also be considered. Discuss timing and options with your OB and genetic counselor once pregnancy is confirmed.
How is the mosaic percentage determined?
PGT-A next-generation sequencing analyzes the DNA from biopsied cells. The proportion of abnormal chromosomes signal in the sample determines the mosaic percentage. Different labs may use slightly different technical protocols that affect exact percentages.
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