Down syndrome: A common birth defect that is usually due to an extra chromosome 21 (trisomy 21). Down syndrome causes mental retardation, a characteristic facial appearance, and multiple malformations. It occurs most frequently in children born to mothers over age 35. It is associated with a major risk for heart problems, a lesser risk of duodenal atresia (partially undeveloped intestines), and a minor but significant risk of acute leukemia. Treatment for Down syndrome includes early intervention to develop the mental and physical capacities to their utmost, speech therapy, and surgery, as needed, to repair malformations. About one-half of children with Down syndrome have heart defects, most often holes between the two sides of the heart (septal defects). With appropriate intervention, most children with Down syndrome live active, productive lives into at least middle age. Most are mildly to moderately retarded, although some have IQs in the low'normal range. Unfortunately, most adults with Down syndrome eventually develop Alzheimer's disease as they grow older. Down syndrome was also once called mongolism, a term now considered out of date, as the disorder has no relationship to Mongolian or Asian heritage. It can occur in any racial or ethnic group.