trisomy 18

n. A genetic disorder caused by the presence of three copies of chromosome 18 attributed to the nondisjuncture of chromosomes 17 and 18 in an egg or sperm cell. Most fetuses who have this abnormality spontaneously abort, and those that are born seldom live more than a few weeks. They usually have a small, abnormally shaped head with a small mouth and jaw, low-set ears, clenched fists with overlapping fingers, and a rounded foot bottom. Also called Edwards syndrome.