Embryo Mosaicism: What It Means

Learn what mosaic embryos are in IVF, how they affect success rates, UK clinic guidelines, and the complex choices they present.

WHEN IVF GETS COMPLICATED

8/16/20251 min read

When embryos are tested during IVF (often through PGT-A), most will be reported as either euploid (chromosomally normal) or aneuploid (abnormal). But sometimes, results fall into a grey area - mosaic embryos. These embryos contain a mix of normal and abnormal cells, making their potential more complex to predict.

What Causes Mosaicism?

Mosaicism occurs when an error in chromosome separation happens in one of the embryo’s early cell divisions. This means:

  • Some cells have the correct number of chromosomes.

  • Some have missing or extra chromosomes.

It’s not yet fully understood how often mosaicism occurs, but some studies suggest it may be present in 10–20% of embryos tested through PGT-A.

Can Mosaic Embryos Lead to a Healthy Baby?

Yes - but the success rates are generally lower than for euploid embryos. Possible outcomes include:

  • Healthy pregnancy and birth (many mosaic embryos self-correct as they develop).

  • Failed implantation.

  • Early miscarriage.

The risk depends partly on:

  • The proportion of abnormal cells (low vs high mosaic).

  • Which chromosome(s) are affected.

UK Guidelines on Mosaic Embryo Transfer

In the UK, the HFEA advises that mosaic embryos can be considered for transfer when no euploid embryos are available, after thorough counselling. Clinics must:

  • Explain the potential risks and uncertainties.

  • Recommend prenatal testing if pregnancy occurs.

  • Keep clear records of the outcome.

Ethical Considerations

Choosing whether to transfer a mosaic embryo can be emotionally and ethically challenging. Couples must weigh:

  • The chance of a healthy pregnancy.

  • The possibility of passing on genetic issues.

  • The emotional impact of uncertain outcomes.

Success Rates

Studies show:

  • Low-level mosaics (20–40% abnormal cells) have better success rates than high-level mosaics (>40%).

  • Implantation and live birth rates are still lower than with euploid embryos.

However, many healthy babies have been born from mosaic embryos - making them an important option for patients with few or no normal embryos.

Bottom Line

Mosaic embryos are not automatically “bad embryos.” With careful consideration, they can offer a chance of pregnancy when options are limited. Decisions should be made in close consultation with your clinic, ideally with genetic counselling.