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Autism Risk Lowest when Pregnancies Spaced 2 to 5 Years Apart

Large study supports earlier research associating closely spaced pregnancies with increased risk of autism
September 29, 2014


(Sept 29, 2014) A new study finds strong evidence that pregnancies spaced two to five years apart have the lowest risk that a child will develop autism spectrum disorder (ASD).

The findings support earlier research associating closely spaced pregnancies with higher risk for ASD. The new study is the first to likewise associate widely spaced pregnancies with increased risk. The report appears in the October issue of the Journal of the American Academy of Child and Adolescent Psychiatry.

"It’s important to realize that we can't say from this study that spacing of pregnancies, per se, is a cause of ASD,” says study leader Keely Cheslack-Postava, of Columbia University. “The importance of this finding lies in the clues that it can provide in terms of understanding how the prenatal environment is related to outcomes after birth.”

“Nutritional depletion is a potential explanation for the effect of shorter intervals between pregnancies,” comments epidemiologist Michael Rosanoff, Autism Speaks director for public health research. “This is in line with studies suggesting that depleted folic acid or iron during pregnancy may increase autism risk.” Rosanoff was not directly involved in the study. “The increased risk associated with longer pregnancy intervals is a new finding that needs additional investigation to better understand,” he says.

The new report comes out of the larger Finnish Prenatal Study of Autism. The researchers analyzed national health records from 7,371 Finnish children born between 1987 and 2005. A third of the children had a diagnosis of autism. The analysis compared their medical records to those of other children born at similar times and locations.

Compared to children conceived 24 to 59 months after their next-oldest sibling:

* Those conceived after less than 12 months were 50 percent more likely to be diagnosed with autism.

* Those conceived after more than 60 months were 30 percent more likely to be diagnosed with autism.

* Those conceived after more than 120 months were over 40 percent more likely to be diagnosed with autism.

The researchers adjusted for other factors that might explain the association. This included parent age and prior number of children.

"This study provides further evidence that environmental factors during or near the prenatal period play a role in autism,” says senior study author Alan Brown, also of Columbia University. Scientists use the term “environmental factors” to refer to a broad range of risk factors beyond inherited genes. The environmental factors that research has most-strongly associated with increased autism risk are maternal infection during pregnancy, birth complications and advanced parental age at time of conception. Conversely, research suggests that mothers who take prenatal vitamins before conceiving reduce the odds that their children will develop autism.

Also see "Risk vs Cause in Autism," by pediatric neurologist Martha Herbert.