The main reason is the high rate of aneuploidy that has been increased as the maternal age increases, this can happen in the oocyte or the embryo, and cause a low rate of pregnancy, miscarriages and a decrease in the live birth rate. Most of these chromosomal abnormalities are generated in the oocyte, around between 10-30 percent of fertilized eggs are aneuploid, with more than 50% in women over 40 years. In oogenesis it can occur during meiotic divisions, and can also occur during mitotic divisions of the embryo. Being the abnormalities during meiosis those that affect the oocyte, therefore, the integrity of the embryo, and the mitotic ones affect the embryo, being able to self-correct or giving mosaic embryos, this means presence of cells with chromosomal content normal and abnormal. Genetic recombination, an incorrect assembly of the meiotic spindle, a loss of cohesions over time, a low level of mitochondria or a shortening of telomeres, are factors that may be involved in the increase of aneuploidies. Reactive oxygen species can even be involved in many of these processes.
