If the findings hold up in subsequent papers, head circumference could one day serve as an early warning sign of a child at risk of autism, he added.
"Head circumference is a relatively simple, noninvasive measurement that can be done during well-child visits and during clinic visits," Horrigan said. "It could emerge as a potential screening tool during the first year of life, especially when working with a child of a high-risk family who already has a child with an autism spectrum disorder."
In the study, researchers analyzed medical records that included head circumference measurements during checkups at pediatricians' offices for 180 children from birth to 18 months. Head circumference, while not a direct measurement of brain size, is a proxy for brain size, according to the study. Children also had an MRI to measure brain size at age 3 years.
The study included 61 children with regressive autism, 53 kids with early-onset autism and 66 typically developing children.
Overall, the children with autism had a larger head circumference than those without. But that was mainly explained by the 10 percent of children -- all boys, and all with regressive autism -- who had very large heads, or heads larger than 97.5 percent of typically developing children, Amaral said.
When children with early-onset autism were separated out, their head and brain size was similar to that of typically developing kids.
In the study, about 53 percent of the children with autism had the regressive form, Amaral said.
The study does have limitations, noted Horrigan, including: the MRIs were done at only one point in time; the study included a relatively small number of participants; and the head circumference data were retrospective, or had been collected previousl
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