Researchers have long thought that autism was more common among white children in the U.S. than other ethnic or racial groups. But a new study of birth records in highly diverse Los Angeles County suggests that children whose mothers emigrated from certain countries may be particularly at risk.
Children of foreign-born mothers who are black, Central or South American, Vietnamese or Filipino were more likely to be diagnosed with autism disorder than children of white U.S.-born mothers, according to the study, published online Monday in the journal Pediatrics.
The study also found that children of Hispanic and African-American mothers born in the U.S. have a higher risk of autism than white children whose mothers were born in the U.S.
The risks were adjusted for maternal age, education, health insurance and other factors known to influence the diagnosis rate.
"Once we adjusted for all these variations, the patterns looked completely different," said Beate Ritz, an epidemiologist at the University of California, Los Angeles, and the study's senior author.
The study authors said the findings could fuel growing speculation among some researchers that autism may be triggered by stress or other problems in prenatal development, when brain connections are first forming.
Diagnosis of autism, which involves a wide range of intellectual, social and language difficulties, has been rising in the U.S. for reasons that aren't clear. The Centers for Disease Control and Prevention recently estimated that 147 out of 10,000 children-about 1 in every 68-were diagnosed with autism spectrum disorders by age 8.
The UCLA researchers focused on autistic disorder, a narrower and more severe form, diagnosed in 21 children per 10,000 nationwide.
Looking at birth records of 1.6 million children born in Los Angeles County between 1995 and 2005, they identified 7,540 who were diagnosed with autistic disorder between ages 3 and 5, and found information on their mothers' race, ethnicity and place of birth on their birth certificates.
The risk of autistic disorder was 76% higher in children of black foreign-born mothers, compared with children of white, U.S.-born mothers. It was 43% higher in children of mothers from Vietnam, 26% higher in children of mothers from Central or South America and 25% higher in children of mothers from the Philippines.
The risk was about 30% lower among children whose mothers were born in China or Japan.
Children of Hispanic mothers born in Mexico had a lower risk of autism, although the risk was about 13% higher among mothers of Hispanic origin and African-American origin born in the U.S.
Similar patterns held true when the researchers looked at autistic disorder coupled with mental retardation, language difficulties and emotional outbursts.
Information about autism rates in Latin America and South East Asia is limited, the study noted. But Dr. Ritz speculated that many of the immigrant women who gave birth in Los Angeles during the study period may have had a history of trauma, violence and nutritional deficiencies in their own lives that affected how their bodies handled stress during pregnancy.
Immigrant mothers also may be exposed to unfamiliar viruses in the U.S. that can affect a fetus's development, Dr. Ritz said.
Social factors may play a role in higher autism rates as well. Unlike many parts of the country, Los Angeles County offers free assessment and services for children with autism regardless of income, insurance coverage or immigration status. Word-of-mouth in ethnic and immigrant communities could influence diagnosis rates, Dr. Ritz said.
Michael Rosanoff, associate director of public health research for the nonprofit advocacy group Autism Speaks, said that while the study provided more data, it didn't shed much light on what is causing the higher rates of diagnosis.
"A next step would be to look at these individual children and speak to these parents about what happened during pregnancy and immigration to determine some potential risk factors," he said.
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