trisomy 18 a condition in which a baby is conceived with three copies instead of the normal two copies of chromosome #18. Children with this condition have multiple malformations and mental retardation due to the extra chromosome #18. Some of the problems include: low birth weight, small head, small jaw, malformations of the heart and kidneys, clenched fists with abnormal finger positioning, and malformed feet. The mental retardation is severe. Ninety five percent of children with this condition die before their first birthday.