Mosaic Embryo Management in Thailand PGT Cycles: What Patients Need to Know

What Is a Mosaic Embryo?

In preimplantation genetic testing for aneuploidy (PGT-A), a mosaic embryo is one that contains a mixture of cells with normal (euploid) chromosome numbers and cells with abnormal (aneuploid) chromosome numbers. Unlike a uniformly euploid embryo (all cells normal) or a uniformly aneuploid embryo (all cells abnormal), a mosaic embryo shows intermediate results. The percentage of abnormal cells—reported as the mosaicism level—can vary from low (e.g., 20–30%) to high (e.g., 50–80%).

Mosaicism can arise from errors during cell division after fertilization. It is important to understand that PGT-A tests only a few cells biopsied from the trophectoderm (the part that becomes the placenta), so the result may not perfectly represent the inner cell mass (which becomes the fetus).

How Do Thai Clinics Report Mosaic Results?

Thai fertility centers that offer PGT-A typically use next-generation sequencing (NGS) to analyze embryo biopsies. NGS can detect mosaicism with varying sensitivity. When a mosaic result is identified, the laboratory report usually includes:

• The specific chromosome(s) involved
• The estimated percentage of abnormal cells (e.g., 30% mosaic for chromosome 21)
• A classification such as low-level or high-level mosaicism

Clinics may follow guidelines from professional societies (e.g., Preimplantation Genetic Diagnosis International Society, PGDIS) that recommend considering transfer of low-level mosaic embryos when no euploid embryos are available. However, each clinic may have its own policy. Patients should ask their doctor how the lab reports mosaicism and what thresholds are used for classification.

Options for Mosaic Embryos

When a PGT-A cycle yields only mosaic embryos (or a mix of euploid and mosaic), the following options are typically discussed:

1. Transfer of a Mosaic Embryo

Some clinics may offer transfer of a low-level mosaic embryo after thorough counseling. The rationale is that many mosaic embryos can self-correct in utero or result in healthy live births. However, the risk of an abnormal pregnancy outcome (e.g., miscarriage, birth defects, or a child with a chromosome disorder) is higher than with a euploid embryo. The decision depends on the specific chromosome, the mosaicism level, and the patient’s history and preferences.

2. Discard or Not Transfer

High-level mosaic embryos or those involving chromosomes known to cause severe disorders (e.g., trisomy 13, 18, 21) are often not recommended for transfer. Some patients may choose to discard such embryos or use them for research.

3. Further Testing

In some cases, additional testing such as preimplantation genetic testing for structural rearrangements (PGT-SR) or comprehensive chromosome screening may be considered if the mosaicism involves a structural abnormality. However, re-biopsy and re-testing of a mosaic embryo is rarely performed because it may damage the embryo and results may still be inconclusive.

4. Additional IVF Cycle

If no euploid embryos are available and the patient does not wish to transfer a mosaic embryo, another IVF cycle may be recommended to try to obtain euploid embryos.

Counseling and Decision-Making

Thai clinics typically provide genetic counseling before and after PGT-A. When a mosaic result is obtained, a follow-up counseling session is essential. Topics covered include:

• The meaning of mosaicism and its limitations
• Estimated risks for pregnancy and child health
• Options for prenatal diagnosis (chorionic villus sampling or amniocentesis) if pregnancy occurs
• Possibility of confirmatory testing after birth

Patients should be aware that no guarantee can be given about the outcome of a mosaic embryo transfer. Each case is unique, and decisions should be made in close consultation with the fertility team.

Regulatory and Ethical Considerations in Thailand

Thailand does not have specific laws governing the transfer of mosaic embryos. The decision is left to the clinic’s ethical committee and the patient’s informed choice. Some clinics may have a policy against transferring any mosaic embryo, while others may consider it on a case-by-case basis. International patients should confirm the clinic’s policy before starting a cycle.

Questions to Ask Your Thai Clinic

• How does your lab report mosaicism? What threshold is used for low vs. high level?
• Do you offer transfer of mosaic embryos? Under what conditions?
• What counseling is provided before a mosaic embryo transfer?
• What are the estimated risks for the specific mosaic result?
• Is there an option for additional testing or a second opinion?

Summary

Mosaic embryo management in Thailand PGT cycles involves careful interpretation of NGS results, individualized counseling, and shared decision-making. While transfer of low-level mosaic embryos is possible in some clinics, it carries higher risks than euploid embryos. Patients should seek clear communication from their clinic and consider all options, including additional IVF cycles, before making a decision.

Frequently asked questions

Can a mosaic embryo result in a healthy baby?

Yes, many mosaic embryos can self-correct or result in healthy live births, but the risk of an abnormal outcome is higher than with a euploid embryo. The specific chromosome and mosaicism level affect the prognosis.

Do all Thai clinics transfer mosaic embryos?

No, policies vary. Some clinics may transfer low-level mosaic embryos after counseling, while others may not transfer any mosaic embryos. Patients should confirm the clinic's policy before starting treatment.

What is the difference between low-level and high-level mosaicism?

Low-level mosaicism typically refers to 20–40% abnormal cells, while high-level mosaicism is >40–50%. Higher levels are associated with greater risk of an abnormal pregnancy.

Is prenatal testing recommended after a mosaic embryo transfer?

Yes, if pregnancy occurs, prenatal diagnosis (chorionic villus sampling or amniocentesis) is recommended to confirm the fetal chromosome status.

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.