trisomy 18
a congenital disorder in which a person has a third copy of chromosome 18 instead of the usual two copies. It is characterized by low birth weight with microcephaly, various facial anomalies, a prominent occiput, overlapping of the index finger over the third finger, and visual abnormalities. Less than 10% of children live past their first year, and these children usually have severe intellectual disability. Also called Edwards syndrome; E trisomy.