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- The majority of cases of mental retardation are classified as familial retardation, in which there is a history of retardation within the family.
- In nearly one-third of the cases, there is an identifiable biological reason for mental retardation.
- The most common biological cause of retardation is Down syndrome.
Down syndrome occurs due to the presence of an extra chromosome.
- Fetal alcohol syndrome (caused by a mother's use of alcohol while pregnant) also can produce retardation.
- In other cases of mental retardation, an abnormality occurs in the structure of a chromosome.
- Birth complications, such as a temporary lack of oxygen, may also cause retardation.
- In some cases, mental retardation occurs after birth, following a head injury, stroke, or infections such as meningitis.
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