The clinical consequences of mosaicism depend on which chromosome is involved, and when and where an error occurs. Mitotic rescue of a meiotic error or a very early mitotic error will typically lead to general mosaicism while a mitotic error at a specific cell lineage point typically leads to confined mosaicism. The clinical consequences of mosaicism are dependent on numerous aspects, with the consequences being unique for each event.

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Comprehensive chromosome screening is typically used for aneuploidy analysis of blastocysts. It is believed that either day of blastocyst development is acceptable. Euploidy rates and outcomes were examined between day 5 and day 6 blastocysts in two studies. First, euploidy rates of day 5 and day 6 blastocysts were examined on a per-embryo and per-patient basis. Second, outcomes were compared when only euploid day 5 or day 6 blastocysts were transferred in a cryopreserved embryo transfer cycle. In cycles (n = 70) that had blastocysts biopsied on both day 5 and day 6, day 5 blastocysts had a higher chance of being euploid than day 6 blastocysts (125/229 [54.6%]) and (77/180 [42.8%]), respectively (P = 0.0231). Similarly, euploid rates in blastocysts from patients (n = 193) with day 5 biopsy, day 6 biopsy, or both, were significantly higher in day 5 (235/421 [55.8%]) compared with day 6 (184/413 [44.6%]) blastocysts (P = 0.0014). In the second study, 50 women (36.1 ± 4.3 years) and 39 women (35.1 ± 3.8 years) with only euploid day 5 or euploid day 6 blastocysts transferred during a cryopreserved embryo transfer had similar cycle outcomes. Although underpowered, these data suggest that euploid day 6 blastocysts are as capable of positive outcomes as their euploid day 5 counterparts.

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