By Amy Norton

NEW YORK (Reuters Health) - Severe stress during pregnancy has been proposed as a risk factor for autism, but a new study finds that at least one source of such stress appears unrelated to the disorder.

The study, which analyzed records on 1.5 million children born in Denmark, found no evidence of an increased autism risk among children whose mothers lost a close family member shortly before or during pregnancy.

The findings, reported in the journal Pediatrics, suggest that mothers' bereavement -- as an indicator of substantial stress -- does not contribute to autism risk.

However, the researchers say, the study does not prove that prenatal stress plays no role in autism development.

While genetics are thought to be important in autism risk, experts also believe that environmental factors are involved. Just what those factors are remains unclear. In theory, severe stress during pregnancy could affect fetal brain development in a way that raises the risk of future autism.

Accurately measuring prenatal stress is "very difficult," Dr. Jiong Li, the lead researcher on the current study, told Reuters Health.

"A simple indicator of stress, like bereavement in our study, may fail to unveil the association between stress and autism," explained Li, a researcher at the University of Aarhus in Denmark.

In addition, Li pointed out, bereavement around the time of pregnancy is fairly infrequent, which makes it more difficult for a study to detect a general effect.

The findings are based on records for nearly 1.5 million children born between 1978 and 2003. More than 37,000 of those mothers lost a child, spouse, parent or sibling during pregnancy or in the year before becoming pregnant. There was no evidence that their children had a higher risk of developing autism.

"What we can say is, our data do not support a strong association, and people should not be panic about such an event," Li said.

"But," the researcher added, "our data would not argue against the evidence from other studies that severe stress could affect neurological development in fetus."

SOURCE: Pediatrics, April 2009.

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