Updated July 22, 2013.

A study at the University of Washington pinpoints rapid head growth in infants as one significant marker for autism:

• Children with autism have normal-size heads at birth but develop accelerated head growth between six and nine months of age, a period that precedes the onset of many behaviors that enable physicians to diagnose the developmental disorder, according to new research from the University of Washington's Autism Center.
  The study also indicates that this aberrant growth is present in children who have the early onset form of autism as well as those later diagnosed with the regression type of the disorder, according to Sara Webb, who led the research.
  
  "We know there are a number of risk factors for autism, and if we can pinpoint them we have better ways of identifying children at risk so we can get them into prevention or monitoring," said Webb, a UW research assistant professor of psychiatry and behavioral sciences.
  
  "This abnormal or accelerated rate of head circumference growth is a biological marker for autism. It occurs before the onset of behavioral symptoms at 12 months of age such as a child's failure to respond to their name, a preoccupation with certain objects, not pointing to things, a lack of interest in other people and the absence of babbling.
  
  "By itself, head growth is not an indicator of autism," she said, "because kids are going to be getting bigger and development is so variable. However, if you notice it and some of these other symptoms, it is a red flag to seek evaluation."
  

It's important to note that many children with autism DO babble, DO point, and ARE interested in people, but may exhibit idiosyncratic speech patterns or other symptoms not mentioned in the press release. This can make it very tough to know whether a child is really exhibiting developmental delays, or is simply developing a little differently from the norm.
Since head growth is typically measured in well-baby visits, and there are standards by which head growth is measured, this particular marker may be unusually helpful to parents concerned about their child's development. Pediatricians are often loathe to suggest developmental delays or recommend evaluation for developmental issues based on criteria such as eye contact or speech development, since even typical children develop these traits and abilities so differently and at such different times. Head growth, however, is a straightforward and concrete measure that is either typical or atypical, and as a result may provide a more direct route to developmental evaluation and, if necessary, early intervention.

_References:

Dawson et al. Accelerated head growth can predict autism before behavorial symptoms start. Journal of Child Neurology, February 2007.