If you have received a PGT result describing an embryo as “mosaic,” you are likely facing a decision that feels both technical and personal. A mosaic result means the laboratory found a mix of cells with different chromosome patterns in the same embryo. It is not the same as a fully normal or fully abnormal result. In Thailand, as elsewhere, mosaic reporting and transfer policies vary between clinics and laboratories. This guide explains what mosaic results can mean, why counseling matters, and how to prepare questions before deciding whether to transfer.
At a glance
- Mosaic means an embryo has a mixture of cells with different chromosome findings.
- Mosaicism exists on a spectrum; “low-level” and “high-level” are laboratory descriptions, not guarantees about outcome.
- PGT-A, PGT-M, and PGT-SR are different tests with different purposes and limitations.
- Reporting thresholds and transfer policies vary by laboratory and clinic.
- Counseling should cover uncertainty, alternatives, and what is known versus unknown.
- No test can guarantee an embryo will implant, develop normally, or result in a healthy child.
What “mosaic” means in a PGT report
Preimplantation genetic testing (PGT) involves taking a small sample of cells from an embryo, usually at the blastocyst stage, and analyzing them. The result is based on that sample, not on every cell in the embryo. When the laboratory reports mosaicism, it means the sample contained cells with different chromosome results. For example, some cells may appear to have a normal number of chromosomes while others show an extra or missing chromosome.
Mosaicism can occur naturally in early embryo development. It can also reflect technical factors in the biopsy or analysis. Because the sample is small, the reported percentage is an estimate of the proportion of cells with the variant, not a precise measurement of the whole embryo.
Low-level versus high-level mosaicism
Laboratories may describe mosaicism as low-level or high-level based on the estimated percentage of affected cells. These categories are not standardized across all laboratories. A result labeled “low-level” at one lab might be reported differently at another. The clinical significance of any mosaic result depends on multiple factors, including which chromosome is involved, the percentage reported, and the clinic’s experience.
It is important not to interpret a low-level mosaic result as a guarantee of a healthy outcome, nor a high-level result as a guarantee of a poor one. The evidence continues to evolve, and uncertainty remains.
PGT-A, PGT-M, and PGT-SR: different tests, different questions
PGT is an umbrella term. The type of test matters for interpreting a mosaic result.
- PGT-A looks at the number of chromosomes (aneuploidy screening). Mosaic results are most commonly discussed in the context of PGT-A.
- PGT-M looks for a specific inherited condition that runs in the family. It is not a general chromosome screening test.
- PGT-SR looks at structural chromosome rearrangements, such as translocations, that may affect reproduction.
Each test has limitations. None can detect every possible genetic or chromosomal issue. None can guarantee a successful pregnancy or a healthy child. PGT is a tool for information, not a promise.
Why mosaic reporting varies
Mosaic reporting is not uniform. Laboratories use different thresholds to classify an embryo as normal, mosaic, or abnormal. Some may report a percentage range; others may use categories. The biopsy technique, the number of cells analyzed, and the analysis platform can all influence the result.
In Thailand, as in other countries, there is no single national standard that all clinics follow for mosaic reporting. This means the same embryo might receive a different description at a different laboratory. When you receive a mosaic result, ask your clinic to explain how their laboratory defines and reports mosaicism.
What a mosaic result does and does not tell you
A mosaic result tells you that the tested sample contained a mix of cells with different chromosome findings. It does not tell you:
- Whether the embryo will implant.
- Whether a pregnancy would continue normally.
- Whether a child would be affected by a chromosome condition.
- How the untested cells in the embryo are arranged.
Some mosaic embryos have resulted in healthy births. Others have not implanted or have been associated with pregnancy loss. The outcome depends on many factors, including the specific chromosome involved and the proportion of mosaic cells. Because of this uncertainty, decisions about mosaic embryo transfer are often described as trade-offs rather than clear right or wrong choices.
Counseling questions after a mosaic result
Good counseling is essential. It should help you understand your specific result, the limits of the evidence, and your options. Consider asking your clinic or a genetic counselor:
- What exactly does my report say, and how does your laboratory define mosaicism?
- Which chromosome is involved, and what is known about mosaicism for that chromosome?
- What percentage of cells was reported as mosaic, and how was that estimated?
- What are the possible outcomes if we transfer this embryo?
- What are the alternatives, such as transferring a different embryo, another cycle, or choosing not to transfer?
- What additional testing or monitoring would you recommend during pregnancy if we transfer?
- Can we speak with a genetic counselor or review the original laboratory report together?
Take notes. Ask for the information in writing if possible. It is reasonable to request time to consider your options.
Transfer trade-offs: what to weigh
Deciding whether to transfer a mosaic embryo involves weighing several factors. There is no single correct answer for everyone.
- Uncertainty: The outcome cannot be predicted with certainty. Some patients are comfortable with this uncertainty; others prefer to avoid it.
- Alternatives: Do you have other embryos available? Are you willing to undergo another stimulation cycle? What are the emotional, physical, and financial implications?
- Chromosome involved: Some chromosomes are associated with different levels of risk than others. Ask your clinic to explain what is known about your specific result.
- Monitoring: If you transfer, what prenatal testing or monitoring is available and acceptable to you?
- Values and priorities: What matters most to you in this decision? How do you weigh the possibility of a healthy birth against the possibility of loss or a child with a chromosome condition?
Some clinics may offer transfer of mosaic embryos with additional counseling and monitoring. Others may have policies that differ. Ask your clinic about their approach and the reasoning behind it.
Questions to ask your clinic before deciding
Use this checklist to guide your conversation:
- Can you explain my PGT report in plain language?
- What is the laboratory’s definition of low-level and high-level mosaicism?
- What is known about outcomes for mosaicism involving this specific chromosome?
- What are the alternatives to transferring this embryo?
- What monitoring or prenatal testing would you recommend if we transfer?
- What support is available if the transfer does not result in a live birth?
- Are there any costs or logistical steps we should plan for?
You may also want to ask for a referral to a genetic counselor who can review the result independently.
Next steps
After receiving a mosaic result, take these steps:
- Request a copy of the full laboratory report.
- Schedule a counseling session with your clinic or a genetic counselor.
- Write down your questions and bring them to the appointment.
- Ask about the clinic’s experience with mosaic embryo transfers.
- Consider the alternatives and what they would mean for you.
- Take time to make a decision that aligns with your values.
For more general information about PGT and embryo transfer in Thailand, you can explore our PGT in Thailand and embryo transfer in Thailand guides. You can also browse our guides and FAQ for additional context.
Frequently asked questions
What does a mosaic embryo result mean?
A mosaic result means the laboratory found a mix of cells with different chromosome patterns in the small sample taken from the embryo. It is not a diagnosis of a specific condition, and it cannot predict whether the embryo will implant or result in a healthy birth. The significance depends on factors such as the chromosome involved and the percentage of mosaic cells reported.
Is low-level mosaicism safe for transfer?
There is no universal answer. Some clinics may offer transfer of low-level mosaic embryos with additional counseling and monitoring, while others may have different policies. The evidence is evolving, and outcomes are not guaranteed. Discuss your specific result with your treating clinic and consider speaking with a genetic counselor.
How is mosaicism reported in Thai PGT laboratories?
Reporting practices vary. Laboratories may use different thresholds to classify an embryo as normal, mosaic, or abnormal, and there is no single national standard. Ask your clinic to explain how their laboratory defines and reports mosaicism, and request a copy of the full report.
What questions should I ask after a mosaic PGT result?
Ask which chromosome is involved, what percentage of cells was reported as mosaic, how the laboratory defines mosaicism, what the possible outcomes are, what alternatives exist, and what monitoring would be recommended if you transfer. It is also reasonable to ask for a referral to a genetic counselor.
Can a mosaic embryo result in a healthy baby?
Some mosaic embryos have resulted in healthy births, but outcomes cannot be predicted. A mosaic result does not guarantee a healthy baby, nor does it guarantee a poor outcome. The decision to transfer involves weighing uncertainty, alternatives, and your personal values with guidance from your clinic.
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Medical information notice: This article is educational and does not replace individual assessment, diagnosis, genetic counselling or treatment advice from a licensed clinician. Provider services, availability, fees and policies should be verified directly before booking.