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Children who have autism usually don't get diagnosed before the age of four. But a study published in the current issue of The New England Journal of Medicine suggests the disorder starts well before birth. The findings should bolster efforts to understand how genes control brain development and contribute to autism. NPR's Jon Hamilton reports.

JON HAMILTON, BYLINE: About halfway through pregnancy, the brain of a fetus starts to get organized. Eric Courchesne, an autism researcher at UC San Diego, says this process is especially important in the cortex, a thin sheet of cells that's critical for learning and memory.

ERIC COURCHESNE: This sheet is like a layer cake. There are six layers, one on top of the other. In each layer, there are different types of brain cells.

HAMILTON: Courchesne suspected that these layers might be altered in the brains of children with autism. So he and a team of researchers studied samples of cortex from 22 children who had died. The cortex came from areas known to be associated with the symptoms of autism. Courchesne says some of the samples came from typical children, others from children with the disorder.

COURCHESNE: In autistic cortex, there are patches about five millimeters to 10 millimeters in diameter in which specific cells in specific layers seem to be missing.

HAMILTON: In these patches, Courchesne says, instead of distinct layers, there are disorganized collections of brain cells without clear boundaries. He says this almost certainly means that something went wrong very early in brain development.

COURCHESNE: Somewhere between around 19 weeks gestation and about 30 weeks of gestation, the cortex begins to develop clear distinct layers. So something must have gone wrong at or before that time.

HAMILTON: Courchesne says these patches of disorganized cortex would have different effects on the brain, depending on where they occur and how many there are. That could help explain why the symptoms of autism vary so much. Courchesne says it also suggests how a child's brain can compensate for the damage by rewiring.

COURCHESNE: There may be some cortical areas that are less affected. And therefore, rewiring around these patches may, in some individuals, be successful leading to improved functions. That's one of our guesses about how it is that autistic children with treatment very commonly get better.

And it suggests that treatment should start early in childhood in order to encourage as much rewiring as possible. The new study appears to confirm research from UCLA showing that people with autism tend to have genetic changes that could disturb the formation of layers in the cortex.

HAMILTON: Stan Nelson, a psychiatrist at UCLA, says the study also adds to the considerable evidence that autism starts in the womb.

DR. STAN NELSON: The overwhelming set of data is that the problems are existing during brain development, probably as an embryo or fetus.

HAMILTON: But Nelson says some of the new study's findings are surprising and even a bit perplexing. For example, he says, it's odd that only certain bits of brain tissue contain these disorganized cells.

NELSON: Why are we just seeing them in small patches? Why is the whole cortex not disorganized?

HAMILTON: Nelson says it's also odd that 10 of the 11 children with autism had the same sort of disorganized patches. He says that's not what you would expect with a disorder that's known to involve many different genes, presumably affecting many different aspects of brain development. So he'd like to know what researchers would find if they looked at hundreds of brains instead of just a few.

NELSON: What fraction of all the kids with autism are going to have these small patches? I think the jury's out on that.

HAMILTON: The study's authors agree. But they say the patches may turn out to be a common feature of autism. Finding out for sure will be difficult. That's because this sort of research can't happen unless parents of children who die agree to donate their child's brain to science.

Jon Hamilton, NPR News.

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