ATLANTA  — Autism cases are on the rise again, largely because of wider screening and better diagnosis, according to federal health officials.

The rate of U.S. cases of autism and related disorders rose to about 1 in 88 children. The previous estimate was 1 in 110.

The new figure is from the latest in a series of studies that have been steadily increasing the government’s autism estimate. This new number means autism is nearly twice as common as officials said it was only five years ago, and likely affects roughly one million U.S. children and teens.

“We’re not quite sure the reasons for the increase,” said Coleen Boyle of the Centers for Disease Control and Prevention.

Autism is diagnosed by making judgments about a child’s behavior; there are no blood or biologic tests. For decades, the diagnosis was given only to kids with severe language and social impairments and unusual, repetitious behaviors. The definition of autism has gradually expanded, and autism is now shorthand for a group of milder, related conditions, including Asperger’s syndrome.

The CDC study is considered the most comprehensive U.S. investigation of autism prevalence to date. Researcher gathered data from areas in 14 states — Alabama, Arizona, Arkansas, Colorado, Florida, Georgia, Maryland, Missouri, New Jersey, North Carolina, Pennsylvania, South Carolina, Utah and Wisconsin.

They looked specifically at 8-year-old children because most autism is diagnosed by that age. They checked health and school records to see which children met the criteria for autism, even if they hadn’t been formally diagnosed.

An earlier report based on 2002 findings estimated that about 1 in 150 children that age had autism or a related disorder such as Asperger’s. After seeing 2006 data, the figure was revised to about 1 in 110. The estimate released Thursday, based on 2008 data, is 1 in 88.

The study also found that autism disorders were almost five times more common in boys. And that an increasingly large proportion of children with autism have IQs of 85 or higher — a finding that contradicts a past assumption that most autistic kids had IQs of 70 or lower.

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.

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.