(HealthDay News) -- Researchers have identified thousands of types of abnormalities present at birth that can cause physical or mental disability. But all the variations can be put into a few categories, according to the U.S. National Institute of Child Health and Human Development.
Structural birth defects, according to the institute, are physical problems such as cleft lip or cleft palate, heart defects (missing or misshaped valves, for instance) and abnormal limbs (such as club foot). They also include problems related to the growth and development of the brain and spinal cord -- neural tube defects such as spina bifida.
Functional birth defects, it says, are problems that stem from a body part or body system that does not work properly, often leading to developmental disabilities. They include:
Nervous system or brain problems such as learning disabilities, mental retardation, behavioral disorders, speech difficulties, convulsions and movement troubles. Examples: autism, Down syndrome, fragile X syndrome.
Sensory problems such as blindness, cataracts, hearing difficulties and deafness.
Metabolic disorders that involve a body process or chemical reaction, such as conditions that limit the body's ability to rid itself of waste material or harmful chemicals. Example: hypothyroidism.
Degenerative disorders, which cause the person's health to steadily worsen with age. Examples: muscular dystrophy, Rett syndrome.
Sometimes a combination of factors is involved. Birth defects can, in fact, lead to both structural and functional problems.