Genetics appear to play an important role in autism. See, for example, http://www.ncbi.nlm.nih.gov/pubmed/18414403. Yet, a number of alternative theories exist (most lacking in evidence) that purport to explain the origin of autism. For example, viral infections (particularly rubella); the vaccine for measles, mumps, and rubella (MMR); thimerosal (a mercury preservative used in some vaccines); parental age; folate deficiency or other nongenetic one-carbon metabolism disturbances; hypocholesterolemia; exposure to prenatal testosterone; and, hyperactivation of the immune system. The reason such often unsupported theses may obtain a following is the urban myth that the rate of autism is rapidly increasing. As noted in a prior post, the so-called increase may have more to do with improved methods of assessment and changes in classification of disorders than with an actual increase.
Recently published research shows a significant statistical effect that arises from the taking of prenatal vitamins in the three-month period prior to conception and one-month period after conception. Women who took such vitamins during this time-frame reduced the risk of autism by 40%. The reduction was especially significant in women with genetic variants that impact folate metabolism (folate is a significant component of prenatal vitamins).
The authors note that replication of their results is important to confirm the result found in their study. Since the results are epidemiological in nature, they also note the need for establishing a biological mechanism to explain the effect that was apparent statistically.
The study can be found at http://journals.lww.com/epidem/Abstract/2011/07000/Prenatal_Vitamins,_One_carbon_Metabolism_Gene.8.aspx.