How should mosaic embryos be prioritised and counselled according to the latest evidence?

[Verified against ESHRE PGT good practice recommendations 2020] The honest starting point, in the guideline's own words: the clinical significance of transferring mosaic embryos is currently unknown. That is not a placeholder pending better data — it is the current position, and it should be conveyed to patients as uncertainty rather than dressed as a risk-stratified algorithm. What ESHRE actually requires: Each centre should decide whether or not to report mosaicism, based on internal validation and recent literature. Reporting is therefore a centre-level decision, and practice legitimately differs between clinics. The centre's policy on identification and transfer of embryos with mosaicism or segmental aneuploidy must be documented and shared with the patient during genetic counselling. Where such an embryo is transferred and pregnancy results, appropriate follow-up should be arranged. Transfer could potentially carry a risk of first-trimester miscarriage or of viable unbalanced offspring. Technical limits that constrain any prioritisation scheme: Sensitivity and specificity of mosaicism detection are specific to each platform, protocol and bioinformatics pipeline, and cannot be exchanged among platforms. A percentage-based threshold validated on one system does not transfer to another. With few cells in a biopsy it is almost impossible to detect changes representing less than 20–30% of the sample. Apoptotic or dead cells in the biopsy can generate profiles resembling mosaicism, as can cells in S-phase. The biopsy may also simply be non-representative of the embryo. So a ranking scheme built on mosaicism percentage is more precise than the underlying measurement supports. If your centre uses one, it should be based on your own validation data and presented to patients with its limitations.