A rare chromosomal abnormality in which there is a trisomy of chromosome 18. This inherited condition is also called Edward's syndrome. It is characterized by severe mental retardation, a small head with malformed ears set low, and a face with a pinched appearance. Other likely defects include a cleft palate, webbed hands, missing thumbs, clubfeet, as well as heart and genitourinary defects. Individuals with trisomy 18 usually die within a few months after birth.