Edwards syndrome: Trisomy 18 syndrome. Children with the syndrome have an extra chromosome 18 with a characteristic pattern of multiple malformations and mental retardation. Features include 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 with an IQ too low to measure. Nineteen out of 20 of these children die before their first birthday. The condition is named after the British physician and geneticist John Edwards who discovered the extra chromosome in 1960.