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Genetically abnormal human oocytes are more common than we thought

A current study published in the American Journal of Human Genetics, carried out by a team at Washington State University has shown that around 7% of the time oocytes have an unbalanced number of chromosomes. This is due to failures in recombination during the process of meiosis: when genetic material from the maternal and paternal copies of each chromosome are exchanged before cell division, to produce oocytes with half the genetic material.

'We have known for a long time that advancing maternal age increases the likelihood of chromosomally abnormal eggs, but this observation demonstrates that many chromosome errors have nothing to do with maternal age,' says Professor Terry Hassold, team leader of the study. 'They are, instead, errors that are extremely common in our species, for reasons that are unclear.

As expected, after fertilization, chromosomally abnormal oocytes can result in embryos with unbalanced numbers of chromosomes. This is something that can be analysed nowadays thanks to some techniques used in assisted reproduction.

Preimplantation Genetic Testing Aneuploidy (PGT-A) is a test used for IVF procedures in which the embryo´s genome is analysed by NGS screening. This shows whether each embryo has an unbalanced number of chromosomes (aneuploidies) or not, in order to select the most viable one and increase probability of implantation.

Bioarray pioneered the application of NGS to PGT-A in 2014 and is since then providing this analysis to European and South American IVF centres, with great clinical outcome.