Researchers at the University of Washington's Autism Center in Seattle said the this abnormal or accelerated rate of head circumference growth is a biological marker for autism.

Study leader Sara Webb said by itself, head growth isn't an indicator of autism, because children's growth and development are variable. However, if a parent notices larger head growth and some of other autism symptoms, such as a child's failure to respond to his or her name, a preoccupation with certain objects, not pointing to things and a lack of interest in other people -- it is a red flag to seek evaluation, Webb advised.

The data in the study, published in the Journal of Child Neurology, involved three measurements made during the first three years of life. The researchers used the medical records of 28 boys who had been diagnosed with autism spectrum disorder between the ages of 3 and 4 and eight boys with developmental delay.

Infant head measurements are typically done on a regular basis by pediatricians through the first 18 months of life but aren't reliably done after that, Webb said.