Most causes of mental retardation are present at birth (congenital). Some of these are major causes. Others are quite rare or only occasionally lead to mental retardation. Some of the major congenital conditions associated with mental retardation include:

• Down syndrome. A common genetic disorder that is among the most common causes of mental retardation. It is also characterized by slow growth and abnormal facial features. Heart defects are also common. Down syndrome is more common in children born to older mothers.
  
  
• Fragile X syndrome. A common genetic disorder that is a major cause of mental retardation in boys. It affects boys more frequently and more severely than girls. Other characteristics include shyness and social anxiety, learning disabilities, and autistic features (e.g., poor eye contact, odd movements).
  
  
• Fetal alcohol syndrome. A collection of birth defects and other problems caused by maternal alcohol consumption during pregnancy. Characterized by a low birthweight, abnormal facial features, behavioral problems and delayed development, including mental retardation.
  
  
• Cerebral palsy. A condition that causes brain damage and affects muscle control. The degree of impairment varies greatly among patients. Mental retardation and other neurological disorders (e.g., seizures) also occur in over half of all patients with the condition.
  
  
• Phenylketonuria. A metabolic abnormality in which protein is not properly processed and builds up in the bloodstream. It leads to mental retardation if left untreated, but early treatment with a special diet can be used to prevent brain damage.

Other conditions that often result in mental retardation, but are usually rare include:

• Miller-Dieker syndrome. A disorder characterized by an abnormally smooth brain surface (lissencephaly). This tends to result in severe mental retardation and other developmental problems, seizures and abnormal facial features.
  
  
• Rubinstein-Taybi syndrome. A genetic condition associated with moderate to severe mental retardation. Other characteristics include short stature, abnormal facial features, and broad thumbs and first (“big”) toes.
  
  
• Johanson-Blizzard syndrome. A rare genetic disorder that typically causes moderate mental retardation. However, some patients may have mild retardation or normal intelligence. Other characteristics include an abnormal nose that is small or “beak shaped,” tooth abnormalities, low birthweight and problems with digestion, the pancreas and the thyroid.
  
  
• Williams syndrome. A rare genetic condition associated with mild to moderate mental retardation and learning disorders. It is characterized by abnormal facial features that may be described as “elfin-like,” low muscle tone (hypotonia), loose joints, poor growth during infancy and early childhood, and heart and blood problems.
  
  
• Prader-Willie syndrome. A rare genetic disorder characterized by obesity due to constant, insatiable hunger. It is also associated with mental retardation and decreased muscle tone.
  
  
• Hunter syndrome. A rare metabolic disorder that only affects boys. It causes certain enzymes to build up and damage body tissues. It is characterized by abnormal facial features, mental retardation, hyperactivity, stiff joints, enlarged organs (e.g., liver, spleen) and hearing loss.
  
  
• Noonan syndrome. A genetic disorder that causes abnormal development. It is associated with abnormal facial features, mental retardation, short stature, delayed puberty and congenital heart disease.
  
  
• Thrombocytopenia absent radius (TAR) syndrome. A rare genetic disorder caused by low platelet levels (thrombocytopenia). It is characterized by potentially severe bleeding, which is worst during infancy. It often causes mental retardation, possibly due to bleeding in the brain. Other characteristics include underdevelopment or absence of the bones in the forearms, congenital heart disease and kidney defects.
  
  
• Dubowitz syndrome. A very rare genetic disorder with a wide range of signs and symptoms that may vary widely in severity between patients. Common signs and symptoms include mental retardation (usually mild), short stature, abnormal facial features, small head size and eczema.
  
  
• Ohdo blepharophimosis syndrome. A condition characterized by an inability to open the eye fully (blepharophimosis) and drooping eyelids. It is also associated with developmental delays, including mental retardation, tooth abnormalities, deafness, and congenital heart disease.
  
  
• Smith-Lemli-Opitz syndrome. A condition that occurs primarily in Caucasians. It is characterized by abnormal facial features, a small head size, mental retardation or learning disabilities, behavior problems, low muscle tone, and defects in the heart, lungs, kidneys, digestive tract and/or genitalia.
  
  
• Shprintzen syndrome. Also called velocardiofacial syndrome, this genetic condition is characterized by abnormalities in the back of the mouth and top of the throat, which cause speech problems. It is also associated with developmental delays, including mental retardation, congenital heart disease and abnormal facial features.

Other conditions that may occasionally result in mental retardation include:

• Adams-Oliver syndrome. A rare condition that affects the scalp and cranium. It is associated with limb abnormalities and, occasionally, mental retardation.
  
  
• Oculoauriculovertebral dysplasia. A rare genetic disorder with widely varying signs and symptoms that may occur in some patients but not in others. Some abnormalities in the cheeks, jaw, mouth, ears, eyes, and/or spinal column are usually present. They may occur on both or only one side of the body. Mild mental retardation may also be present.
  
  
• Weaver syndrome. Another condition characterized by rapid growth. It is associated with increased muscle tone (hypertonia), exaggerated reflexes (spasticity), foot deformities, and developmental delays, which may include mental retardation.
  
  
• Asymmetric crying facies syndrome. A condition caused by an underdeveloped facial muscle that causes the right and left side of the face to be misbalanced and look unmatched (asymmetry), particularly when smiling or crying. It is often associated with congenital heart disease.
  
  
• Beckwith-Wiedemann syndrome. A rare disorder that causes rapid growth. It is characterized by large body size, a large tongue, enlarged organs and kidney problems.
  
  
• Chondroectodermal dysplasia. Also called Ellis-van Creveld syndrome, this rare form of dwarfism is most common among the Pennsylvania Amish. It is characterized by short forearms and lower legs, extra fingers and toes (polydactyly), abnormal nails and teeth and heart defects.
  
  
• Pallister-Hall syndrome. A wide-ranging disorder than may be very mild and nearly unnoticeable or severe and life-threatening. When mild, it is associated with extra fingers or toes and bifid epiglottis (split in the tissue that blocks food from entering the windpipe). When severe, it is characterized by a split in the windpipe, resulting in death shortly after birth. More moderate forms may lie anywhere between these two extremes.