Trisomy 18, is a chromosomal abnormality in which the affected child has an extra chromosome 18. trisomy 18 is usually associated with severe multiple malformations and is usually fatal, so it rarely survives to 1 week of age. Patients have trisomy 18 in all or most of their cells, mostly all trisomies, due to chromosome non-separation during meiosis. Patients have a short life span, with 30% dying within 1 month of birth, 50% dying within 2 months, and 10% living to 1 week of age but with severe mental retardation.