What are mosaic embryos?

A mosaic result means the biopsied cells were not all the same — some had a normal chromosome count and some did not. Mosaicism is a normal biological phenomenon in early human embryos rather than a defect peculiar to your embryos. As an embryo divides, occasional errors occur, so a mixture of cell types is common. What makes this difficult is that the biopsy takes only a few cells, from the outer layer that becomes the placenta. A mosaic result tells you those particular cells were mixed. It cannot tell you what proportion of the rest of the embryo is affected, or whether the part that becomes the baby is affected at all. Two things worth understanding about the result itself: Not every clinic reports mosaicism. ESHRE states that each centre decides whether to report it, based on its own validation. So the same embryo could be reported as mosaic at one laboratory and euploid at another. That is not an error by either — it reflects genuinely different reporting thresholds. The measurement has limits. Detecting changes present in less than roughly 20 to 30% of a small cell sample is very difficult, and cells damaged during biopsy can produce results that look like mosaicism when it is not there. Ask your clinic what their policy is on reporting and transferring mosaic embryos. ESHRE says that policy should be documented and shared with you during counselling.