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Mosaicism in embryos

Mosaicism in embryos

As an embryo divides after fertilisation, mistakes in cell division sometimes produce abnormal cells with an incorrect number of chromosomes. These abnormal cells can be detected using preimplantation genetic screening (PGS) if they are included for testing after a biopsy is performed. When abnormal cells are detected alongside normal cells the embryo is called ‘mosaic’.

Mosaic embryos present a challenge for doctors because there is still very little data concerning the impact of mosaicism on embryo viability. To date, the outcome of only a few hundred mosaic embryo transfers have been reported worldwide. So far, the research literature shows that mosaic embryos have a lower implantation rate and higher miscarriage rate, but some are still capable of resulting in a healthy live birth with no evident issues at all. While it is still not clear how this occurs, it is thought that in ongoing pregnancies, the abnormal cells are outcompeted by the normal cells present in the embryo. Since a biopsy represents only 5 cells of a 100+ cell embryo, we don’t actually know how many normal and abnormal cells are in the rest of the embryo.

For these reasons, deciding what to do with mosaic embryos can be very difficult and can depend on many other aspects, including whether other embryos are available, and risk factors such as which chromosomes are involved. Flinders Fertility continually reviews research is this area so we can provide our patients with the latest information in order to help make decisions regarding these embryos, and will refer patients for a genetics consultation when necessary.

Victor, A R et al. 2019 ‘One hundred mosaic embryos transferred prospectively in a single clinic: exploring when and why they result in healthy pregnancies’, Fertility and Sterility, vol.111, no.2, pp. 280–293.
Grati, F R et al. 2018 ‘An evidence-based scoring system for prioritizing mosaic aneuploid embryos following preimplantation genetic screening’, Reprod Biomed Online,, pp 442-449.