Experts found that children whose mothers had taken the vitamins were around half as likely to develop autism as those whose mothers took none.
But the team cautioned that although they had found a possible association, more research was needed.
Writing in the British Medical Journal (BMJ), experts from universities and hospitals in Philadelphia, Stockholm and Bristol, examined data for 273,107 mothers and their children living in Stockholm.
All the children were aged four to 15 by 31 December 2011, and were born between 1996 and 2007.
The woman's use of folic acid, iron, and multivitamin supplements had been recorded at their first antenatal visit.
Cases of autism spectrum disorder were also identified in national registers.
After adjusting for factors that might influence the findings, experts found that women who took multivitamins, with or without additional iron and/or folic acid, had a lower chance of their child developing autism with intellectual disability compared to mothers who took none.
When iron and folic acid were looked at alone, there was no consistent evidence that their use was associated with a reduced risk of autism.
NHS guidelines say a healthy diet should give women most of the vitamins and minerals they need in pregnancy.
But women should also take 400 micrograms (mcg) of folic acid every day for the first 12 weeks of pregnancy and 10 micrograms (mcg) of vitamin D a day, they say.
More than one in 100 people are on the autism spectrum, including an estimated 700,000 people in the UK.
There are many different characteristics that make up autism, but people may be under or oversensitive to sounds, touch, tastes, smells, light and colours.
They may also find social situations challenging and experience "meltdowns" when anxious.
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