"What we've discovered is really very startling – the first glimpse of when autism begins," said Eric Courchesne, director of the Center for Autism Research at Children's Hospital San Diego and a UCSD professor of neurosciences.

If the finding is verified by more research, he said, it could prompt clinicians to routinely measure head size during the first year of infancy – years before autism is usually diagnosed – so behavior modification can begin sooner.

Studies show that the earlier steps are taken, the better their outcomes.

Courchesne's report, co-authored by Ruth Carper and Natacha Akshoomoff of UCSD and Children's Hospital, is published in today's Journal of the American Medical Association.

Autism has long been associated with abnormal brain structure and larger head circumference in children, but this study was the first to identify possible early-warning signs.

However, the cause of abnormal brain growth patterns remains elusive.

The San Diego researchers also hope their work will help other researchers target genetic abnormalities or contaminants such as pollutants or food additives consumed during pregnancy that might trigger the sudden brain growth they found in the children they studied.

Autism spectrum disorder, a range of symptoms from mild to severe disability, affects one in every 160 people and is growing worldwide. In California, 21,000 people are diagnosed with true autism – the most severe form – double the number in 1999.

The disorder was characterized by Dustin Hoffman in the movie "Rain Man." He played an autistic savant who was wrapped so tightly in a world of numbers and ritual that he couldn't function normally.

Most cases are far more severe, with patients demonstrating severe and erratic responses to sounds, sights and smells. Other characteristics can include poor eye contact, rocking movements and aggression to changes in routine.

The team studied 48 children with autism spectrum disorder when they were 2 to 5 years old. The childrens' heads had all been measured and their brains imaged by magnetic resonance at birth and during their first year of life.

What the team found was that the childrens' average head size at birth was smaller than 75 percent of healthy newborns.

Then, starting at about 2 months old, their heads and brains experienced sudden brief bursts of growth. By about 14 months, their average size was larger than 85 percent of healthy children.

"Those that had the greatest acceleration of head growth also had the most severe autism," Courchesne said.

Brain growth then slowed so that by age 5, brain size resembles that of healthy children, but the damage was done.

"There appears to be two phases in autism at the beginning of life. One is brain undergrowth at birth, and the second is that sudden brief burst before the child is 1 year old," Courchesne said.

After that second phase, the first subtle signs of autism emerge.

"During the first four months, babies are smiling and cooing and showing social interactions – not like autism at all," he said. "But sometime between 4 and 12 months of age, there's less social responsiveness and they begin showing all the signs of autism."

Courchesne's research was funded by a grant from the National Institute of Neurological Disorders and Stroke.

Some autism experts expressed skepticism.

In an accompanying editorial, Janet E. Lainhart of the University of Utah School of Medicine wrote that it is unclear that accelerated brain growth precedes autism.

"Recognition of the disorder is not always coincident with its onset," she said, adding that when it begins and its cause are unknown.

Dr. Randi Hagerman, a University of California Davis pediatrician and autism researcher, applauded the study's findings.

But autism, she said, is probably caused by abnormalities in some 15 or more genes, which in turn could be caused by environmental contaminants such as industrial chemicals affecting the fetus.

"It still could be environmental causes that stimulate enhanced brain growth," Hagerman said.

The growth abnormalities occur when brain organization and neuronal connections shape the basis of human function and intellect.

Courchesne said if his theory is verified, it might end fears that certain vaccines such as for measles, mumps and rubella, usually given after a child is 1, are the cause of autism.

It could be, he said, that brain growth anomalies are provoked by "genetic factors acting alone or in concert with environmental factors" during fetal development.

The team cautioned that large head circumference doesn't automatically signal autism. Large head size occurs in 6 percent of healthy developing infants and is associated with a number of other serious medical conditions such as tumors and subdural hematomas.

But 59 percent of children later diagnosed with autism are found to have heads roughly 10 percent larger than normal.

"Now we have a trajectory and that's tremendous. We can see the brain changes from being small at birth to larger later," said Dr. Marsha Herbert, pediatric neurologist at Massachusetts General Hospital in Boston.

"But," she said, "you can't judge from this what's causing it. Now we need to keep our minds open about what this really means."