Trisomy 18 is a relatively common syndrome affecting approximately 1 out of 3,000 live births, and affecting girls more than three times as often as boys. It is caused by the presence of an extra number 18 chromosome, which leads to multiple abnormalities. Many of these abnormalities make it hard for infants to live longer than a few months.
Trisomy 18 is a chromosomal condition related to chromosome 18.
People with trisomy 18 have additional DNA from chromosome 18 in some or all of their cells. The extra material disrupts the normal course of development, causing the characteristic features of trisomy 18.
Trisomy 18 results when each cell in the body has three copies of chromosome 18 instead of the usual two copies. A small percentage of cases occur when only some of the body's cells have an extra copy of chromosome 18, resulting in a mixed population of cells with a differing number of chromosomes. Such cases are sometimes called mosaic trisomy 18.
Very rarely, a piece of chromosome 18 becomes attached to another chromosome (translocated) before or after conception. Affected people have two copies of chromosome 18, plus extra material from chromosome 18 attached to another chromosome. With a translocation, the person has a partial trisomy for chromosome 18 and often the physical signs of the syndrome differ from those typically seen in trisomy 18.
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