From VOA Learning English, this is the Health & Lifestyle Report.

Autism spectrum disorder is a developmental disability. People with autism have trouble communicating and with social skills. The U.S. Centers for Disease Control and Prevention says the person also might repeat some behaviors and not want change in their daily activities. Some people with the condition need a lot of help. Others need less.

CDC officials say autism affects one in every 68 children in the United States. More boys than girls are believed to have the condition. But the number of cases appears to be growing. It is unclear whether the growing number shows a real increase or comes from more knowledge about this disorder.

Symptoms of autism

Common signs of autism include trouble making eye contact and a delay in learning how to speak. Some people with severe autism never learn how to talk. Many people with autism also have difficulty understanding facial expressions and the feelings of others. They also have trouble making friends of the same age.

Doctors have learned how to recognize autism, but much is still unknown about its causes.

Researchers at Harvard University have come closer to finding answers. They found that women exposed to the highest levels of fine particulate air pollution late in their pregnancies are two times more likely to give birth to a child with autism. The findings appeared in Environmental Health Perspectives -- a publication of the National Institute of Environmental Health Sciences.

Marc Weisskopf led the Harvard researchers. Mr. Weisskopf studies environmental conditions that cause sickness, developmental disabilities and deaths. He and his team studied women who were late in their pregnancies, shortly before they gave birth. In all, they examined medical records from all 50 U.S. states for about 116,000 mothers and their children.

The study found that the women who were around high levels of fine particulate matter air pollution were at highest risk of having an autistic child. The increased risk of these women was two times that of women who lived in areas with low levels of fine particulate pollution.

The researchers found that the timing of exposure to pollution was important. They found no increased risk of autism in children whose mothers were around high levels of pollution before becoming pregnant. And the study found air pollution does not seem to increase the risk of children developing autism after they are born.

Marc Weisskopf says this finding does not prove there is a direct link between pollution and autism. But, he adds, it has ratcheted up, or increased, his team's confidence that there is a relationship between the two.

"Finding an association like this that's very specific in time rules out a lot of other possible explanations for that. So, it really ratchets up the strength of our confidence that we've got something really related to the air pollution here."

What can pregnant women do?

Mr. Weisskopf says pregnant women should avoid air pollution as much as possible. But he warns that other things may increase the risk of autism spectrum disorder.

"You can avoid being in extremely polluted cities during pregnancy if possible. You can also choose to go running in a park rather than next to a street. But that said, I think also it's very important to recognize that autism spectrum disorders is a very multi-factorial disorder. And there are lots of reasons why risk could be increased."

Autism is believed to result from a combination of environmental and genetic factors.

Autism Speaks is an organization that helps families with autistic children. Paul Wang is head of medical research at Autism Speaks. He says the findings are very compelling. He notes that during the last three months of pregnancy, the brain of the fetus develops very quickly. He says the study does not change his group's main suggestion: focus on treatment and educational therapies.

"For parents of children who have already been diagnosed, the focus should continue to be on treatment for them, on the behavioral, educational therapies that are available and that we know can help kids who are diagnosed with autism."

For now, Marc Weisskopf says researchers are trying to identify the exact substances in air pollution that increase the risk of autism.

And that's the Health and Lifestyle Report from VOA Learning English. I'm Anna Matteo.

This story was based on a report from VOA's Jessica Berman. Anna Matteo wrote it for VOA Learning English. George Grow was the editor.

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autism spectrum disorder - n. any of a group of developmental disorders (as autism and Asperger's syndrome) marked by impairments in the ability to communicate and interact socially and by the presence of repetitive behaviors or restricted interests

spectrum - n. a continuous sequence or range

fine particulate – n. very small solid material or liquid in the air

compelling – adj. capable of causing someone to believe or agree

ratchet up – v. to cause to move by steps or degrees -- usually used with up or down

confidence – n. the feeling of being certain that something will happen or that something is true

therapy – n. the treatment of physical or mental illnesses