A large study in Norway has found that the early timing of such supplements is critical.
Taking folic acid (Vitamin B9) in early pregnancy is known to protect against spina bifida and other neural tube defects in children.
The Norway study of more than 85,000 babies born between 2002 and 2008 found that mothers who took folic acid supplements in early pregnancy had a 40pc reduced risk of having children with autistic spectrum disorder compared to the mothers who did not take the supplements.
The reduction in the risk was noticed particularly in those who too folic acid from four weeks to eight weeks after the start of the pregnancy.
The researchers found that the timing appeared to be a critical factor.
They also analysed whether the risk of autistic disorder was influenced by the use of other dietary supplements but did not find any link between fish oil supplements or other vitamins and minerals and the condition.
The study was a collaboration between the Norwegian Institute of Public Health and Columbia University in New York.
It is published in the Journal of the American Medical Association.