Study Summary
2 minute readMosaic embryos (where the biopsy found a mix of normal and abnormal cells) have long been treated as a problem. Many clinics refused to transfer them, or discarded them alongside fully abnormal embryos. This study pooled the outcomes of 1,000 mosaic embryo transfers from centres around the world and compared them with 5,561 euploid (normal) transfers.
The headline: mosaic embryos work. Overall they led to an ongoing pregnancy or birth 37% of the time, versus 52% for euploid embryos. Not as good as normal, but a long way from useless, and far better than throwing them away. The study then showed that not all mosaics are equal, and built a ranking so a clinic can choose which to transfer first.
- Hypothesis: that the specific features of a mosaic result, how many cells are abnormal and what kind of abnormality, predict how well the embryo does, and can be used to rank mosaics for transfer.
- Study Type: This was a large compiled analysis, assembled to have enough transfers to separate mosaic sub-groups by outcome (implantation, and ongoing pregnancy or birth). It is well suited to ranking mosaic types against each other. It can’t prove cause and effect, and it wasn’t built to measure cumulative live-birth rates over several transfers, or to give reliable per-chromosome numbers.
- Conclusion: mosaic embryos deserve their own category. They’re not 'abnormal'. Lower-level mosaics (under 50% abnormal cells) and segmental mosaics (a piece of a chromosome, not a whole one) do best. Complex mosaics (three or more chromosomes) do worst, but even they still produce some healthy births.
Try it with a result
Pick a mosaic type and level to see how embryos like it did in this study, next to the mosaic and euploid averages.
For a fuller picture, use the PGT-A Decoder, which draws on the same dataset, and talk it through with your clinic.
The study
This study picked up where PGT-A leaves off: it followed already-tested blastocysts through transfer, and did not measure the earlier stages.
Study Population
A broad, real-world group: women having IVF with PGT-A across several countries. Their average age was in the mid-30s but ranged well into the 40s, and the single biggest reason for testing was age. So the findings speak directly to women who are testing embryos because of their age and have ended up with mosaic results.
anyone told they have 'only mosaic' embryos left, or advised to discard mosaics.
if you plan to transfer untested embryos, or your clinic is already comfortable with mosaic embryo transfers. It also says nothing about transferring fully aneuploid (uniformly abnormal) embryos.
Methodology
- Every embryo in the study was a normal-looking blastocyst that was biopsied, tested with PGT-A, came back mosaic (or euploid), and was actually transferred.
- For each one the researchers recorded the mosaic level (percentage of abnormal cells) and the mosaic type (a piece of a chromosome, a whole chromosome, or several), then linked those to whether it implanted, became an ongoing pregnancy or birth, or miscarried.
- With 1,000 mosaic transfers they had enough to compare sub-groups against each other and against the euploid benchmark, then to build a transfer-priority matrix from level, type and embryo grade.
What they found (it is not just a ranking)
Mosaics work and should be their own category
37% ongoing pregnancy or birth is not 'abnormal'. Filing mosaics under 'abnormal' leads to discarding viable embryos.
The level of mosaicism impacts pregnancy and birth rates
Among whole-chromosome mosaics, under 50% abnormal cells did far better than 50% or more (ongoing pregnancy/birth was 36% vs 19% for these two categories).
Type matters more than most people realise:
- Segmental mosaics (part of a chromosome) do best: around 44 to 46% ongoing pregnancy or birth - which is close to a modest euploid.
- Single whole-chromosome mosaics sit in the mid-30s.
- Complex mosaics (three or more chromosomes) are lowest at about 21%. The more chromosomes involved, the lower the odds.
Even the worst still works sometimes
The high-level complex group, the bottom of the ladder, still implanted and produced births in some cases. Low odds is not no odds.
Age did not change the odds
Unlike the euploidy rate itself, which falls steeply with age, once you have a mosaic embryo its chance of working did not track with maternal age (they compared under 34 with 34 and over, and the raw data shows no clear decline across the 40s). This separates two things people sometimes confuse:
- how likely you are to make a normal embryo versus
- how well a given embryo performs once you have it.
Any chromosome can still lead to a healthy birth.
In fact, mosaicism in any of the 23 chromosomes produced viable pregnancies in the data (per-chromosome numbers were too small to create a ranking system at that level).
Why does a 'mixed' embryo produce a healthy baby? Evidence to date suggests self-correction is possible as long as some normal cells exist. These normal cells out-compete and crowd out the abnormal ones. Further, the biopsy that is taken for PGT-A comes from the embryo cells that become the placenta, while the cells that become the baby are left untouched. Testing company Igenomix has previously stated that they found in well over 90% of cases, that when they did test the inner cell mass of embryos with under 50% mosaicism, it was euploid. So this suggests placental cells in particular may be more capable of correction as long as the ICM remains euploid.
What it means for older women
The euploidy rate falls with age. But the performance of a mosaic embryo does not. So a woman in her early 40s with a low-level or segmental mosaic is looking at roughly the same odds from that embryo as a younger woman with the same result.
In a situation where every embryo counts, a whole category that was routinely being discarded turns out to be transferable and accompanied by often very positive odds.
Evidence strength
This is compiled, real-world outcome data, not a randomised trial, and known confounders remained. But it is strong enough to rank mosaics and to stop treating them as waste.
Things you can discuss with your doctor
- If you have mosaic embryos, ask your clinic whether they transfer mosaics, and if not, why. Practice varies a lot between labs.
- Ask for the specifics of your mosaic result: the level (percentage) and the type (segmental, single whole chromosome, or complex). Those two facts place your embryo on the ranking.
- Ask how your clinic would order your embryos for transfer. The paper's own ranking tool is public (embryo-score.web.app); it is fair to ask where your embryo sits.
- If you have been advised to discard mosaics, it is reasonable to ask for that advice in light of this data, and to seek a second opinion. Genetic counselling before a mosaic transfer is standard and worth doing.
Source: Viotti M, Victor AR, Barnes FL, et al. Using outcome data from one thousand mosaic embryo transfers to formulate an embryo ranking system for clinical use. Fertility and Sterility, 2021;115(5):1212–1224.
Written by
Michelle Bourke
Founder of Better IVF. I went through IVF over 38 myself, and I write these guides to be the clear, properly-sourced resource I wish I'd had, more honest than the forums and deeper than the clinic pages.