Abstract: Aneuploidy, a numerically chromosomal abnormality, is a major cause of infertility, spontaneous abortion or birth with defects. It occurs at the rate of approximately 5%~7%, 22%~90% and more than 50% in spermatozoa, eggs and early spontaneously aborted fetuses，respectively. The vast majority of frequently observed aneuploidy in humans results from eggs (mothers), and nondisjunction of homologous chromosomes in maternal meiosis I is accused of aneuploid eggs for most chromosomes, except chromosome 13 and 18. Maternal age is the only factor identified epidemically so far for the generation of aneuploid germ cells. Alterations in recombination frequency and location, establishment and maintenance of cohesion between sister chromatids are thought to be responsible for homologous chromosome nondisjunction during meiosis I. Sister chromatids cohesion is established during the last DNA replication before meiosis and homologous chromosomes recombination occurs during meiotic prophase I, both of which happen in fetal ovaries. However, most homologous chromosome nondisjunction takes place in women over the age of 35. This implies that mechanisms incorporating chromosome segregation with recombination and cohesion exist during meiosis, which could be abraded with women aging. Future studies should be focused on what the mechanisms are, and how they work to prevent chromosome missegregation during meiosis.