It is already known that taking folic acid supplements before and during pregnancy reduces a baby's risk of developing neural tube defects (NTDs), such as spina bifida. However, a new study has found that these supplements may also reduce the risk of autism.

Folic acid is a B-group vitamin, which if taken at least eight weeks prior to and 12 weeks after conception, is known to reduce the risk of NTDs by up to 70%. It is found in some foods, such as leafy green vegetables, yeast and liver. However, most pregnant women need to take supplements to reach the recommended daily level of 400 micrograms.

Autism, also known as autism spectrum disorder (ASD), is a biological disorder of the brain that impairs communication and social skills. Signs can include an absence of eye contact, apparent aloofness, avoidance of physical contact even with family, difficulty dealing with interruptions to routine and a lack of interest in other children and what they are doing.

Scientists in Norway looked at over 85,000 children who were born between 2002 and 2008. From these, almost 300 children with autism were identified.

The scientists noted that the use of folic acid supplements in early pregnancy rose significantly during the study period. In 2002, 43% of mothers were taking them, but by 2008, this had risen to 85%.

The study found that women who took the supplements at least four weeks before conception and eight weeks into the pregnancy had a 40% reduced risk of having a child with autism.

The use of folic acid supplements after week 22 of the pregnancy had no effect on the risk of autism. The use of other types of supplements also had no effect.

The study also noted that the reduced risk did not come about if the vitamin was consumed via food.

"It appears that the reduced risk of childhood autism only reflects folic acid supplements, not food or other supplements, and that the crucial time interval is from four weeks before conception to eight weeks into pregnancy," explained lead scientist, Dr Pål Surén, of the Norwegian Institute of Public Health.

He noted that while the study does not prove that folic acid supplements can prevent autism, ‘the findings are so apparent that they constitute a good argument to further examine possible causal mechanisms'.

He added that the findings appear to be related to classic autism only. While a link to Asperger syndrome - a type of autism - was considered, too few affected children were available to study to provide a reliable result.

Details of these findings are published in the Journal of the American Medical Association.