The research, carried out among 85,000 children, found that women who took the supplement, also known to reduce the risk of spina bifida, were 40 per cent less likely to have a child suffering from autism.
The researchers found that timing was key, with folic acid needing to be taken between four weeks before pregnancy to eight weeks into pregnancy to have an impact on autism risk.
But Scottish campaigners said more research was needed before official advice could be given to mothers-to-be.
Folic acid – also known as vitamin B9 – is essential for DNA creation and repair in the human body.
Its naturally occurring form, folate, is found in a number of sources – leafy vegetables, peas, lentils, beans, eggs and liver – but women planning pregnancy are advised to take supplements to a sufficient doseage to protect against spina bifida and similar defects in babies.
The latest study, published in the Journal of the American Medical Association, also found that the supplement could help reduce the risk of autism in young children.
For the research, a team from Columbia University in the US, and the Norwegian Institute of Public Health, studied more than 85,000 babies born in Norway in the largest study of its type.
Mothers were questioned about their diet and families were questioned over three to ten years to measure the development of autism spectrum disorders, including autistic disorder – the most severe form of the condition – and Asperger’s syndrome.
A total of 270 cases of autistic problems were discovered in the children studied. The researchers found that mothers who took folic acid supplements in early pregnancy had a 40 per cent reduced risk of having children with autistic disorder compared to women who did not take folic acid.
Pål Surén, from the Norwegian Institute of Public Health, said: “We examined the rate of autism spectrum disorders in children.
“There was a dramatic reduction in the risk of autistic disorder in children born to mothers who took folic acid supplements.”
The researchers also looked at whether the risk of autistic disorder was influenced by the use of other dietary supplements.
They did not find any association between the mother’s use of fish oil supplements – cod liver oil and omega-3 fatty acids – in early pregnancy and the risk of autistic disorder, and there was no association for the mother’s use of other vitamins and minerals.
Dr Robert Moffat, national director of the National Autistic Society (NAS) Scotland, said: “While we welcome studies such as this, which aim to improve our understanding of autism, far more research would have to be undertaken across a far greater geographical area before any definite conclusions can be drawn from the results. We therefore urge mothers-to-be to treat this research with caution until further investigations are done.”
Ezra Susser, professor of epidemiology at Columbia University’s Mailman School of Public Health, said: “Our findings extend earlier work on the significance of folate in brain development and raise the possibility of an important and inexpensive public health intervention for reducing the burden of autism spectrum disorders.”
In recent years, researchers have explored whether folic acid has other beneficial effects on the development of the brain and spinal cord of unborn children, as well as the proven effects on preventing spina bifida.
A study of language development in 2011 showed that children whose mothers took folic acid supplements in early pregnancy had only half the risk of severe “language delay” at the age of three.