The most common cause of aneuploidies is nondisjunction during meiosis.
Nondisjunction means that a chromosome pair failed to separate during the meiotic division.
This will create one daughter cell with an extra chromosome and another daughter cell with one too few chromosomes. In humans, for example, if nondisjunction of chromosome 13 occurs during first meiotic division, half the gametes will have an extra copy of chromosome 13 and thus contain 24 chromosomes instead of the normal haploid number of 23. The other half of the gametes derived from this primary gametocyte will lack both copies of chromosome 13. Thus, these gametes will only have 22 chromosomes. When these gametes unite with the gamete of the opposite sex, no embryo will be viable.