Mosaic embryos after PGT-A: what the result means, what it does not, and why lab policy matters

Few words on a genetic-testing report cause as much confusion as "mosaic." It sounds precise, as if the lab has counted the normal and abnormal cells in your embryo. It has not. A mosaic result is an inference drawn from a tiny biopsy, and what it means for your embryo is genuinely uncertain.

That uncertainty cuts both ways. A mosaic embryo is not the same as a chromosomally normal one, but it is not automatically unusable either. This guide walks through how the label is produced, what the evidence says about transferring these embryos, and the questions that matter before you decide.

How a "mosaic" result is produced

In genetics, mosaicism means having more than one chromosomally different cell line. PGT-A cannot look at individual cells, though. It analyses DNA from a small group of cells taken from the trophectoderm, the outer layer of a blastocyst that goes on to form the placenta.

According to the ASRM Practice Committee (2023), modern testing measures the amount of DNA for each chromosome in that sample. A result between the values expected for two copies and three copies may be reported as a possible mosaic trisomy. A result between one and two copies may be reported as a possible mosaic monosomy. In other words, the lab sees an in-between signal and calls it mosaic.

Why an in-between signal is not proof

ASRM lists several things besides true mosaicism that can produce an intermediate result:

• ordinary statistical noise and test artefact
• bias introduced when the small DNA sample is amplified
• contamination
• the stage of cell division the sampled cells were in
• how the biopsy was taken, and laboratory conditions

The committee notes that an intermediate result could come from an embryo that is actually normal, actually abnormal, or genuinely mixed. In most cases there is no way to confirm which.

Why the lab you use matters so much

How often embryos are labelled mosaic depends heavily on the laboratory. ASRM reports mosaic results in anywhere from 2% to more than 20% of trophectoderm tests, depending on lab and method. It also notes that the same biopsy might be called "high-level mosaic" by one lab and aneuploid or euploid by another.

The cut-off values a lab chooses, the method it uses, and whether it "masks" low-level findings (not reporting them) all change how many mosaic results appear. Before testing, it is worth asking which lab your clinic sends biopsies to and how that lab reports intermediate findings. Our guide to reading an embryo report explains the terms you are likely to see.

What happens when mosaic embryos are transferred

A substantial body of experience now exists. ASRM's 2023 opinion notes that more than 2,700 embryos with mosaic results had been transferred at the time of writing. The findings so far:

ASRM also notes three case reports of fetal or newborn chromosome abnormalities linked to an earlier mosaic result, and suggests some may have been other kinds of abnormality misread. The reassuring numbers are real, but they come from limited data and a selected group of patients.

Can mosaic embryos be ranked?

Clinics often try to put mosaic embryos in order. ASRM says no universally applicable, evidence-based ranking system exists. Categories may give a rough sense of reproductive potential, but they are not reliable for predicting risk to a pregnancy or a child. The patterns it describes:

• Segmental vs whole-chromosome: a segmental finding (part of a chromosome) is more likely to be a false positive from test artefact. Most prospective studies found higher ongoing pregnancy rates with segmental than with whole-chromosome mosaicism. There is not enough data to say whether fetal risk differs.
• Trisomy vs monosomy: no difference in pregnancy or miscarriage rates was found.
• Several chromosomes involved: mosaicism affecting three or more chromosomes was associated with lower pregnancy potential.

ASRM considers it reasonable to transfer euploid embryos first, depending on the patient's own preferences. Mosaic grading is separate from the visual grade an embryologist gives; our explainer on embryo grading covers that side.

If you are considering transfer

Genetic counselling first

ASRM recommends counselling with a board-certified genetic counsellor who specialises in PGT before any mosaic transfer. That conversation should be clear that no evidence-based method can yet predict adverse outcomes or rank mosaic embryos. The committee also suggests that some patients may find it helpful to speak with a mental health professional while deciding.

Testing during pregnancy

Prenatal options are part of the plan, not an afterthought. ASRM explains:

• Screening tests, including cell-free DNA blood tests, do not diagnose chromosome abnormalities. Cell-free DNA is not designed to detect mosaicism, so a positive screen should be followed by diagnostic testing.
• Diagnostic tests are chorionic villus sampling (at 10–13 weeks) and amniocentesis (from 16 weeks). Both carry a small risk of miscarriage and can miss low-level mosaicism.
• Whether diagnostic testing should be specifically recommended after a mosaic transfer is debated. ASRM notes that ACOG recommends offering it to all pregnant patients.

Clinic policy: ask before you test

Clinics differ. Some transfer selected mosaic embryos, some restrict which categories they will transfer, and some will not transfer any. The ASRM Practice Committee's 2024 PGT-A opinion says clinics should have a written policy on what happens to embryos with abnormal results, and should share it before testing. Consent should cover the possibility of mosaic or segmental results.

This matters most if you might end up with no euploid embryos. Finding out after testing that your clinic will not transfer a mosaic embryo, or will not store it, leaves you with fewer options. If you are having IVF abroad, also ask where embryos would be stored and whether they could be moved to another clinic that has a different policy.

Weighing the decision

There is no single right answer, and people in similar situations choose differently. It can help to separate the decision into parts. How many other embryos do you have, and how many more cycles are realistic for you? How do you feel about a possibly lower chance of implantation, or a higher chance of miscarriage, compared with the cost and strain of another retrieval? And how would you approach prenatal testing if a pregnancy followed?

Some people decide to transfer a mosaic embryo only after any euploid embryos have been used. Others keep it frozen while they gather more information. Either path is easier when the options were understood before testing began.

Questions to ask

• Which genetics lab will analyse the biopsies, and what thresholds does it use to call a result mosaic?
• Does the lab report low-level findings, or mask them?
• Is my result segmental or whole-chromosome, and which chromosome is involved?
• Will you transfer this embryo, and under what conditions?
• Can I see a genetic counsellor who specialises in PGT before deciding?
• What prenatal testing would you suggest if I become pregnant?
• If I decide not to transfer, can the embryo stay in storage while I think?

For an overview of the different kinds of embryo testing and who each one is for, see what PGT is and what it screens for.

Frequently asked questions