Trisomy 18 syndrome: The presence of three copies of chromosome 18, rather than the normal two. Children with trisomy 18 syndrome have multiple malformations and mental retardation. They characteristically have low birth weight, small head (microcephaly), small jaw (micrognathia), malformations of the heart and kidneys, clenched fists with abnormal finger positioning, and malformed feet. The mental retardation is profound, and the IQ is too low to even measure. Nineteen out of 20 children with trisomy 18 syndrome die before their first birthday. Also called Edwards syndrome.