This work forms a basis for future studies aimed at optimizing nuclear and cytoplasmic maturation during in-vitro maturation.

Previous studies assessing the relationship between embryo development, maternal age and chromosome abnormalities were either small or analysed mostly embryos not suitable for replacement. The present study includes >6000 embryos, including many suitable for replacement. Embryos with the best morphology and development were 44% euploid in patients younger than 35, decreasing to 21% in patients 41 and older. The worst morphology group had only 30% normal embryos from patients younger than 35, and 12% in embryos from patients 41 and older. Thus morphological analysis was able to improve the population of normal embryos only from 30 to 44% in the best of cases. Regarding specific abnormalities, 20% of embryos were aneuploid, 32% aneuploid plus other abnormalities, and the rest had post-meiotic abnormalities. Of those, only aneuploidy increased with maternal age. There were no big differences in the frequency of chromosome abnormalities depending on patient indication, within a similar age group. In summary, previous trends detected in suboptimal embryos were also confirmed in the best embryos for replacement. Although dysmorphism and advanced maternal age are both related to chromosome abnormalities, these parameters can yield at most <50% euploid embryos, and other techniques such as preimplantation diagnosis are required to ensure that only euploid embryos are replaced.

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The risk of monoamniotic twinning is increased following IVF/embryo transfer. Zona pellucida disruption appears to increase the incidence of MZ twinning. However, the overall micromanipulation data together with the unexpected placentation data suggest that zona-mediated embryo splitting is not the only mechanism of twinning under artificial conditions.

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