Chromosomal Birth Defects
Two of the most common abnormalities, Down syndrome and Fragile X syndrome, are also frequent causes of mental retardation. Both can be diagnosed before birth. While neither defect is curable, early intervention allows a child to develop to his full potential.
Prevalence: Though Down syndrome occurs in 1 in 800 births overall, the incidence is much higher in older mothers.
Symptoms: A child with Down syndrome generally has characteristic physical features, including slanted eyes; small ears that fold over at the top; a small mouth, which makes the tongue appear larger; a small nose with a flattened nasal bridge; a short neck; and small hands with short fingers.
More than 50 percent of children with this defect have visual or hearing impairments. Ear infections, heart defects, and intestinal malformations are also common among children with this defect.
Though children with Down syndrome have some degree of mental retardation, most can be expected to do many of the same things that any young child can do -- including walking, talking, and being toilet trained -- although generally they learn how to do so later than unaffected children.