How Embryo Mosaicism Affects PGT Results in Thailand

What Is Embryo Mosaicism?

Embryo mosaicism occurs when an embryo contains two or more cell populations with different genetic compositions. For example, some cells may have a normal chromosome count (euploid), while others have an abnormal count (aneuploid). This is different from a uniformly euploid or uniformly aneuploid embryo. Mosaicism can involve whole chromosomes or segments, and the proportion of abnormal cells can vary widely.

In preimplantation genetic testing for aneuploidy (PGT-A), mosaicism is detected when the genetic analysis of a biopsy sample (typically 5–10 cells from the trophectoderm) shows intermediate chromosome copy numbers that do not clearly classify as euploid or aneuploid. The reported level of mosaicism (e.g., low, medium, high) depends on the percentage of abnormal cells estimated by the laboratory.

How Do Thai Labs Report Mosaicism?

Thailand has several IVF laboratories that offer PGT-A using next-generation sequencing (NGS). Most Thai labs follow international guidelines, such as those from the Preimplantation Genetic Diagnosis International Society (PGDIS) or the American Society for Reproductive Medicine (ASRM), to classify mosaicism. However, reporting practices can vary between clinics.

Typically, results are reported as:

• Euploid: No chromosomal abnormalities detected within the sensitivity of the test.
• Aneuploid: Uniformly abnormal chromosome count.
• Mosaic: A mix of normal and abnormal cells, often with a percentage range (e.g., 20–40% abnormal). Some labs further categorize mosaicism as low (20–30%), medium (30–50%), or high (>50%).

It is important to ask your specific clinic how they define and report mosaicism, as thresholds and terminology may differ. Some labs may also report segmental mosaicism (involving only part of a chromosome).

What Does Mosaicism Mean for PGT Results?

A mosaic result adds complexity to embryo selection. Unlike a clear euploid or aneuploid result, a mosaic embryo has an uncertain prognosis. Key points include:

• Not a guarantee of abnormality: A mosaic embryo may still implant and develop into a healthy baby. The abnormal cells may be confined to the placenta or may not affect the fetus.
• Risk of true fetal mosaicism: There is a small chance that the abnormal cell line is present in the fetus, which could lead to a genetic condition. The risk is generally considered higher with higher levels of mosaicism.
• Lower implantation potential: Some studies suggest mosaic embryos have a slightly lower implantation rate compared to euploid embryos, but many have resulted in healthy live births.

Because of this uncertainty, PGT results for mosaic embryos are often reported as “inconclusive” or “intermediate,” and genetic counseling is strongly recommended before transfer.

Transfer Considerations for Mosaic Embryos

Deciding whether to transfer a mosaic embryo involves balancing the chance of a healthy pregnancy against the risk of genetic abnormality. Factors to consider include:

• Level and type of mosaicism: Low-level mosaicism for certain chromosomes may carry lower risk than high-level or complex mosaicism.
• Availability of euploid embryos: If there are euploid embryos available, they are typically prioritized. Mosaic embryos are often considered only when no euploid embryos exist.
• Patient age and reproductive history: Older patients or those with few embryos may be more willing to consider mosaic transfer.
• Genetic counseling: A qualified genetic counselor can help interpret the specific mosaic finding and discuss potential outcomes, including the option of prenatal diagnosis (e.g., chorionic villus sampling or amniocentesis) if pregnancy occurs.

In Thailand, the decision to transfer a mosaic embryo is made jointly by the patient and the treating physician, often after counseling. Some clinics may have specific policies regarding mosaic embryo transfer, so it is essential to discuss this with your clinic beforehand.

Questions to Ask Your Clinic About Mosaicism

To make an informed decision, consider asking your Thai IVF clinic the following questions:

• How does your lab define and report mosaicism? What thresholds do you use?
• Do you offer genetic counseling for mosaic results?
• What is your clinic’s policy on transferring mosaic embryos?
• Can you provide data on outcomes of mosaic embryo transfers at your clinic?
• What follow-up testing (e.g., prenatal diagnosis) is recommended if a mosaic embryo is transferred and results in pregnancy?

Next Steps

If you have received a mosaic embryo result, take the following steps:

1. Request a detailed report from your clinic, including the specific chromosome involved and the estimated percentage of mosaicism.
2. Schedule a consultation with a genetic counselor who can explain the risks and options.
3. Discuss with your fertility specialist whether transfer of the mosaic embryo is appropriate given your individual circumstances.
4. Consider the possibility of additional testing, such as re-biopsy or further genetic analysis, if available and recommended.

Remember that PGT-A is a screening tool, not a diagnostic test. Mosaic results require careful interpretation and should not be the sole factor in decision-making.

Frequently asked questions

Can a mosaic embryo result in a healthy baby?

Yes, many mosaic embryos have resulted in healthy live births. The abnormal cells may be confined to the placenta or may not affect fetal development. However, the risk of a genetic condition is higher than with a euploid embryo, and genetic counseling is recommended.

How is mosaicism detected in PGT-A?

Mosaicism is detected when next-generation sequencing (NGS) of a biopsy sample shows intermediate chromosome copy numbers that do not clearly indicate euploidy or aneuploidy. The estimated percentage of abnormal cells is reported.

Should I transfer a mosaic embryo if I have euploid embryos?

Generally, euploid embryos are prioritized for transfer because they have a higher chance of implantation and lower risk of genetic abnormality. Mosaic embryos are typically considered only when no euploid embryos are available.

Do all Thai IVF clinics report mosaicism the same way?

No, reporting practices can vary. Some clinics may use different thresholds or terminology. It is important to ask your specific clinic how they define and report mosaicism.

What prenatal testing is recommended after transferring a mosaic embryo?

If a mosaic embryo transfer results in pregnancy, prenatal diagnostic tests such as chorionic villus sampling (CVS) or amniocentesis may be recommended to check for fetal mosaicism. Discuss this with your doctor.

Continue your research

• PGT in Thailand overview
• PGT and IVF guides
• Frequently asked questions

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.