In 2013, data from a massive study of more than 85,000 children in Norway suggested that women who take folic acid supplements early in pregnancy lower their risk of having a child with autism. Last month, an analysis of a similarly designed study of more than 35,000 mothers and babies in Denmark found no link between prenatal vitamins and autism risk, raising doubts about the Norwegian finding. Science is always an iterative process, but in the case of pinpointing risk factors for autism, progress has been remarkably slow and difficult. In the past decade, dozens of papers have proposed a vast array of factors that potentially contribute to autism: vitamins such as folic acid, maternal depression and antidepressant use, premature birth, Cesarean birth, advanced paternal and maternal age, overweight parents and exposure to anything from endocrine-disrupting chemicals to air pollutants and pesticides. Some research even suggests that a younger sibling born either too soon or too long after the first child has a heightened risk of autism. All of these are considered environmental risk factors, a term scientists use to refer to anything that isn’t the direct result of a DNA sequence. Almost everyone agrees that autism is caused by a combination of genetics and the environment. But while geneticists can comfortably rattle off lists of dozens of autism-linked genes, there’s much less agreement about which environmental factors contribute to the disorder — and by how much.
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