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In the early 1990s, only a few out of every 10,000 children were diagnosed with the condition, based on some small studies in individual states or cities. But the numbers began to change dramatically after 2000, when Congress directed federal health officials to do more autism research, and CDC started the larger study to see how common autism is.

CDC is also studying the cause of autism, which has remained a mystery.

Genetics is believed to play a role. Some parents and others have believed childhood vaccines trigger autism, even though many studies have not found a connection.

CDC researchers are looking at other possible factors, including illnesses that mothers had while they were pregnant with children who later were diagnosed as autistic. The researchers also are looking into medications that the pregnant women took and those given to their children took when they were young. The first results of that study are expected next year.