Despite more than four decades of research examining how early life experiences may contribute to autism, a new analysis indicates that no risk factor alone increases a child’s odds of having the disorder.

In a review of 40 previous studies, researchers examined how dozens of aspects of a child’s birth and neonatal condition might impact the likelihood of receiving an autism diagnosis later in life.

While some issues — including birth injury, feeding difficulties and low birth weight — did appear to be linked with a greater risk for autism, researchers write in the August issue of the journal Pediatrics that it’s not likely that one risk factor acting alone would lead to an onset of the developmental disorder.

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However, exposure to a number of risk factors could lead to higher chances of autism, they said.

“There is insufficient evidence to implicate any one perinatal or neonatal factor in autism etiology, although there is some evidence to suggest that exposure to a broad class of conditions reflecting general compromises to perinatal and neonatal health may increase the risk,” wrote Hannah Gardener of the University of Miami.

In addition, the research analysis did identify several birth factors with no apparent association to autism, including high birth weight, head circumference and the use of anesthesia.