The gain or loss of one or more chromosomes at meiosis, including trisomy (47 chromosomes) and monosomy (45 chromosomes). Caused by chromosomal nondisjunction.

Failure of chromatids to separate after chromosomal cross-over during meiosis. The origin of aneuploidy. More common in eggs, or oocytes, than sperm, and increases with age in women.

A tiny, compact packet of excess chromosomes discarded first by the primary oocyte as it becomes a secondary oocyte just before ovulation -- the first polar body, with 46 chromosomes; and second by the secondary oocyte immediately after fertilisation (or after activation by other means) -- the second polar body, with 23 chromosomes. The polar bodies lie in the perivitelline space. The chromosome content of each polar body can be tested as part of a preimplantation diagnosis maneuver to infer whether the corresponding oocyte has an aneuploidy: an extra chromosome in the polar body means a chromosome not enough in the oocyte, and vice versa; this is called polar body analysis.

Failure of the ovaries to produce enough follicles, because of a problem in the ovary itself, and resulting in depletion of eggs before the age of 40 years (known as premature menopause, a cause of secondary amenorrhea), or maybe even before the age puberty is expected (causing failure of puberty to happen, including primary amenorrhea). Sometimes occurs in spite of good numbers of primordial follicles that (inexplicably, so far) won't develop. The younger the woman, the more likely that an aneuploidy will be found if a karyotype is done on blood or on a biopsy of the ovary. Estrogen replacement therapy is important to prevent general jeopardy to health, including prevention of osteoporosis.