Autism occurs in one out of every 160 children and is among the more common and serious of neurological disorders of early childhood. While no one knows what causes it, experts agree that diagnosing it early vastly improves the outcome.

Video: New Autism Research RESOURCES Signs In Infants Causes PDD Info Autism Society of America Local ASA Chapter Center for the Study of Autism Child Development Institute Exploring Autism National Institutes of Mental Health

But most of the time, it isn't diagnosed until after the age of 2 or 3, when the child shows difficulty communicating.

In his first few months of life, Brian Guay seemed perfectly normal. It wasn't until after his third birthday that he showed strong signs of autism.

"More and more withdrawn. He also didn't have a tremendous amount of speech and what speech he had he started to lose," Brian's mother Ann Guay said.

But now, researchers believe a simple measurement done during a pediatric checkup may provide an earlier indication of autism.

Researchers from the University of California, San Diego found that abnormally fast growth in the size of an infant's head may be a warning of autism.

In a study of data from 48 autistic children, the researchers found that children who developed autism were born with small heads, but developed bigger heads than 84 percent of other healthy infants during their first year of life. Children who were diagnosed later with the most severe autism had faster head growth than did children with milder forms.

"This burst of overgrowth takes place in a brief period of time, between about two months and six to 14 months of age," said researcher Eric Courchesne (pictured, right), professor of neurosciences and director of the Center for Autism Research at Children's Hospital and Health Center in San Diego. "So, we know it cannot be caused by events that occur later, such as vaccinations for mumps, measles and rubella or exposure to toxins during childhood."

The study is published in Wednesday's issue of the Journal of the American Medical Association.

The researchers said head growth may be an outward sign of autism's brain problems. They said the rapid growth doesn't allow enough time for accumulation of experiences and emotions that guide and shape normal behavior.

"During this period of important learning and plasticity, when the brain is experiencing the world and deciding how to construct itself, it's growing too fast in the infant with autism," Courchesne said. "Without the guidance of experience and learning, the brain may be creating abnormal connections that make it very hard for autistic children to make sense of the world they live in."

But other conditions also can cause rapid head growth, and some babies develop normally. The researchers said the findings must be verified in further studies.

But they said head growth could help doctors diagnose autism early, and early therapeutic services have been shown to produce a better outcome for autistic children.

For Brian, who is now almost 9, specialized education has made a huge difference. While other experts say more research is needed before we know for sure if head size is an indicator of autism, many are excited by the idea.

"I just think that perhaps we could have done more to help Brian sooner, had we known. Now sometimes children find out by 2 and that difference of a year can be critical," Ann Guay said.