Parents often notice the first signs of autism in their children at around 12 to 18 months. Maybe a child isn’t making eye contact, or won’t smile when mom or dad walks in the door.

But a new study suggests there is evidence of autism in the brain even earlier—well before a child’s first birthday—and that the signs can be seen on a magnetic resonance imaging (MRI) scan. “We’re learning that there are biological changes that occur at [the time] or before the symptoms start to emerge,” says Geraldine Dawson, a clinical psychologist and autism researcher at Duke University who was not involved in the new work. “It’s the ability to detect autism at its very earliest stages that’s going to allow us to intervene before the full syndrome is manifest.”

For the study, published this week in Nature, researchers conducted MRI scans on 150 children three times: at six months old, one year and two years. Just over 100 of the children were at high risk because they had an older sibling diagnosed with autism. The faster growth rate of the surface areas of their brains correctly predicted eight times out of 10 which of the high-risk children would go on to be diagnosed with the condition.

Enlargement of the brain seemed to correlate with the arrival of symptoms, says Heather Hazlett, a psychologist at the University of North Carolina’s Carolina Institute for Developmental Disabilities (CIDD), and the paper’s lead author. Still, with only 100 at-risk children, the study is too small to be considered definitive—nor should doctors rush to use MRIs to diagnose autism, Hazlett says.

But if the study results are confirmed in future research, it could offer a new option for screening high-risk children before their symptoms become obvious—and possibly at a time when treatment will be most effective. The faster pattern of brain growth “is a potential biomarker that could be used to identify those infants who perhaps could benefit from early stimulation,” Dawson says. "This could help those children have the best outcomes.”

Autism spectrum disorders—so called because they present a wide range, or spectrum, of different social and communication challenges—are often characterized by behaviors that include rocking motions or obsessions. Parents often do not notice things that would point to an autism diagnosis until about 18 months, when typical children are expected to be talking and interacting socially. By showing the child’s neurobiology changes before behavior does, the study may help parents better understand their child’s experience, says biopsychologist Alycia Halladay, chief science officer at the Autism Science Foundation, a nonprofit that supports research but was not involved in the new work.

By showing scientists more about how brains develop prior to an autism diagnosis, the study may also offer insights into the genetic triggers of autism, says James McPartland, a psychologist at Yale University’s Child Study Center who also did not take part in the research. “When we know more about neural pathways, we can think more about the genetic pathways,” he says.

One thing the study could not show is whether there is anything different about the autism in families that have more than one child with the condition, compared with autism that seems to have no familial connection. One in every 68 children is diagnosed with autism; but among the younger siblings of a child on the spectrum, the rate is as high as one in five.

A number of studies are underway among these younger siblings of children on the spectrum, often called “baby sibs,” who are more likely to develop autism. This group is easier to study than the general population because fewer test subjects are needed to find children who will go on to develop autism. But it is not clear if these “baby sibs” are somehow categorically different from others on the spectrum.

To find enough children to make their study useful, the research team followed more than 500 infants, scanning many of them in the middle of the night so they would be in a deep sleep. It took years to get enough valid data on 150 of them, and the families volunteered their time. “Often we don’t keep in mind the degree of work it takes to do this kind of study,” Dawson says.

Hazlett and senior author Joseph Piven, a colleague at the CIDD, say they began the research about a decade ago, after an earlier study suggested the brains of autistic children were already unusually large by the time they reached their second birthdays—and before autism’s behavioral symptoms usually emerge.

Piven, a psychiatrist, says the mechanism is not precisely clear. But he speculates babies who go on to become autistic experience the world differently in the first year of life than kids who will not have autism do—and that this altered experience of the world may contribute to subsequent brain development in autism.

Dawson says because the brain is changing so much in the first year of life, it may be a critical window of development when behavioral interventions—such as teaching a child to pay attention to a parent’s facial expressions—might have the biggest effect. Because it has not been clear until recently that autistic differences begin as early as pregnancy and infancy, there are no therapies to treat such young children. But such treatments are currently being tested. By giving researchers a potential tool to diagnose babies, Dawson says the new study could open up possibilities for testing the potential new therapies.

McPartland described possible treatments as “hyper-parenting.” Whereas it might be okay to leave a neurotypical child to play with a toy, a child headed for autism might benefit from more interaction, he says, with a parent cooing, laughing and singing. “Supersaturate a child’s environment with social information much as you can,” he says. “And hope that it takes.”