Scientists do not know exactly what causes autism, but they know it most often starts in the first months of life, maybe even before birth. Something goes wrong in the development of a child’s brain, but parents or teachers might not notice it for some years.

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One cause might be a combination of a gene the child inherited mixed with something toxic in the environment, even before birth.

About 15 percent of children with ASD have changes in their genes called mutations. There are more than 100 genes or gene changes that can change the way a child’s brain grows. These changes put the children at risk.

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Here’s how that works: As the unborn baby develops, altered genes might not cause any problem until something else happens. That something could be a virus, a very low birth weight, the mother’s poor health or certain birth problems. The baby’s brain may stop developing the way it should. When something keeps a person from learning and growing the way he should, it is called a developmental disorder. Autism is such a disorder.

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Because of a shocking number of children diagnosed with autism in recent years, a lot of money has been spent on research. That research has found some things that do not cause it.

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You do not get autism from:

• Childhood vaccinations. Nineteen carefully conducted studies of immunized children did not find a link to autism.
   

• Poor parenting. Parents are not to blame.
   

• Another person. You cannot get it by being with someone who has autism.