Aneuploidy is the presence of one extra chromosome (trisomy) or absence of one chromosome (monosomy) in each cell. This leads to different abnormalities such as:

• Trisomy 21- Down syndrome or mongolism
• Trisomy 13 - Patau syndrome
• Trisomy 18 - Edward syndrome.
• Sex chromosome abnormalities
  

In products of conception (tissue from spontaneous abortions) trisomy 16 and trisomy 22 are not so rare.

Aneuploidy can now be rapidly detected by a new cytogenetic technique called Fluorescence in situ hybridization (FISH).