🧬 Mosaic Embryos: Should You Transfer? Latest Research

Until recently, mosaic embryos — embryos that contain a mixture of chromosomally normal and abnormal cells — were considered untransferable. PGT-A laboratories returned these as "abnormal" results, and patients were told to discard them. The clinical landscape changed dramatically between 2020 and 2024 as registry data showed that many mosaic embryos can develop into healthy babies. In 2026, mosaic embryo transfer is a routine consideration in many North American clinics.

This is one of the most rapidly evolving areas of reproductive medicine. Here is what we know now.

What Is a Mosaic Embryo?

When PGT-A (preimplantation genetic testing for aneuploidy) is performed, embryologists biopsy 5 to 10 cells from the trophectoderm of a day 5 or 6 blastocyst. The sample is analyzed by next-generation sequencing (NGS).

Results fall into three broad categories:

• Euploid: all cells in the sample show normal chromosome count
• Aneuploid: all cells show abnormal chromosome count
• Mosaic: a mixture — typically reported when 20 to 80 percent of cells show abnormality

Mosaic embryos are further subcategorized by:

• Low-level mosaic: 20 to 40 percent abnormal cells
• High-level mosaic: 40 to 80 percent abnormal cells
• Segmental vs whole-chromosome mosaicism: partial chromosome changes vs entire chromosome changes
• Specific chromosome involved: some chromosomes (e.g., 21, 18, 13, X, Y) have higher viability implications than others

Why Mosaic Results Occur

Mosaicism typically arises from errors in early embryonic cell division. The biopsy samples only the trophectoderm (placenta-forming cells), not the inner cell mass (baby-forming cells). A mosaic biopsy may not accurately reflect what is happening in the cells that will become the baby — a phenomenon called self-correction or preferential allocation.

The Coticchio and Capalbo Cohort Studies

Large multicenter registries published between 2018 and 2023 by Capalbo, Coticchio, and others have followed mosaic embryo transfers and their outcomes. Key findings:

• Live birth rates for low-level mosaics: roughly 30 to 45 percent per transfer — lower than euploid transfers but substantially higher than zero
• Live birth rates for high-level mosaics: roughly 15 to 30 percent per transfer
• Babies born from mosaic transfers have largely been chromosomally normal at birth, consistent with the self-correction hypothesis
• Miscarriage rates are elevated compared to euploid transfers
• Specific chromosomes matter: certain mosaics carry higher risk than others

The PGDIS 2024 Position Statement

The Preimplantation Genetic Diagnosis International Society (PGDIS) updated its mosaic embryo transfer guidance in 2024. Key points:

• Mosaic embryos may be considered for transfer with appropriate counseling
• Low-level mosaics are generally prioritized over high-level
• Segmental mosaics often have better outcomes than whole-chromosome mosaics
• Specific chromosomes with viable trisomies (such as 21, 18, 13, and sex chromosomes) require especially careful counseling and prenatal testing planning
• All mosaic transfers should be followed with comprehensive prenatal diagnosis (NIPT and offered amniocentesis or CVS)

When Mosaic Transfer Makes Sense

The clinical scenarios where mosaic transfer is most often considered:

• No euploid embryos remaining after all chromosomally normal embryos have been transferred without success
• Only mosaic embryos available from a cycle
• Older patients for whom additional retrievals are unlikely to yield more euploid embryos
• Patients who have completed their IVF journey and want to use every reasonable embryo

When Mosaic Transfer Is Not Recommended

• Euploid embryos still available: euploid embryos should be transferred first
• High-level mosaics involving high-risk chromosomes (especially aneuploidies compatible with live birth such as trisomy 21, 18, 13)
• Patients who have not received thorough genetic counseling

The Counseling Process

Before mosaic transfer, patients should receive:

• Genetic counseling with a certified genetic counselor familiar with PGT-A interpretation
• Discussion of specific mosaic findings in their embryo(s)
• Review of likely outcomes including miscarriage risk and chance of live birth
• Prenatal diagnosis planning, typically including NIPT plus amniocentesis or CVS
• Discussion of decisions that may need to be made if abnormalities are confirmed in pregnancy

What This Looks Like in Practice

Many clinics now have a mosaic transfer policy that defines which mosaic embryos they will transfer and under what conditions. Ask:

• Does your clinic transfer mosaic embryos?
• What is your policy on low-level vs high-level mosaics?
• What chromosomes are considered higher risk?
• What counseling and prenatal diagnosis do you require?
• What live birth rates have you seen with mosaic transfers at this clinic?

A Realistic Frame

Mosaic embryos are no longer treated as "defective" or untransferable. For many patients, particularly those with few or no euploid embryos, a mosaic transfer offers a meaningful chance at live birth with manageable additional risk and clear counseling pathways. For other patients with multiple euploid embryos available, the question of mosaic transfer never comes up.

Confirm with your reproductive endocrinologist and a genetic counselor before deciding on mosaic embryo transfer. To find clinics that offer evidence-based PGT-A and mosaic transfer protocols, browse the Fertility Link Navigator.