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There may be a slightly increased risk of autism for each week a child is born before or after 40 weeks of gestation, according to a new study.
Researchers are still trying to pinpoint the causes of autism, but both genetic and environmental factors are believed to play a role.
Some previous studies have suggested that being born before or after full term (40 weeks) may be associated with an increased risk of autism. But many of those studies were limited in scope and didn't account for sex and birth weight.
In this study, researchers analyzed data on more than 3.5 million children born in Sweden, Finland and Norway between 1995 and 2015. Of those, 1.44% were diagnosed with autism, and 4.7% were born preterm (before 37 weeks of gestation).
The overall risk of autism was low, especially for girls born after 42 weeks of gestation, but the risk increased for each week of gestational age before or after 40 weeks.
Of the children born at term (37 to 42 weeks), 0.83% were diagnosed with autism. The autism rates were 1.67% for those born at 22 to 31 weeks; 1.08% for those born at 32 to 36 weeks; and 1.74% for those born at 43 to 44 weeks.
These differences in risk were independent of sex and birth weight for gestational age, the researchers said. Dr. Martina Persson, an adjunct senior lecturer at the Karolinska Institute in Stockholm, Sweden, led the study.
The findings were published Sept. 22 in the journal PLOS Medicine.
In a journal news release, Persson and her colleagues said the study offers new information about the potential link between autism risk and gestational age at birth -- a factor that's potentially modifiable.
They also said further research is needed to learn more about these possible links and whether they could point to ways to reduce autism risk by addressing preterm birth.
-- Robert Preidt
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