The low reproductive potential of the human species is mainly caused by aneuploidies affecting embryo o fetal development. Although some of these aneuploidies may be paternally inherited or generated mitotically during preimplantation development, the vast majority of aneuploid karyotypes are generated at fertilization as an effect of meiotic errors occurring during the oocyte life cycle. Formation of an aneuploid oocyte derives from chromosome non-disjunction or premature segregation of sister chromatids at meiosis I or II. Less clear is why aneuploidy occurs. Advanced maternal age is strongly positively associated with the prevalence of aneuploidies, including Down syndrome, in spontaneously aborted fetuses and newborns. However, the links that connect maternal age and the cellular mechanisms that are involved in chromosome mal-segregation remain unknown. Factors that may play a role in the generation of aneuploidies are diverse. For example, number and position relative to the centromere of chiasmata formed in the process of recombination during fetal life influence the regularity of chromosome segregation during adult life. Alterations in the profile of the hormonal milieu are also suspected to cause oocyte aneuploidy. Environmental agents and certain lifestyles are believed to be additional factors that expose oocytes to an increased risk of chromosome mal-segregation. Better understanding of the nature and action of these factors could offer future opportunities for preventing at least part of the aneuploidies occurring in the female germ cell.