This study found a link between mother's who took folic acid between 4weeks prior to conception and 8 weeks into their pregnancy and a reduced incidence of severe autism. I've heard before that all women of child bearing age should be taking a pre-natal vitamin and/or folic acid supplements to reduce risk of neural tube defects in case of an accidental pregnancy, but this study suggests it may have even more benefits.

This study can't prove causality yet, but it's definitely exciting research.

Here's a link to the NPR story I heard, which also contains a link to the study itself.
http://www.npr.org/blogs/health/2013/02/13/171828067/folic-acid-for-pregnant-mothers-cuts-kids-autism-risk