Importance  Prenatal folic acid supplements reduce the risk of neural tube defects in children, but it has not been determined whether they protect against other neurodevelopmental disorders.

Objective  To examine the association between maternal use of prenatal folic acid supplements and subsequent risk of autism spectrum disorders (ASDs) (autistic disorder, Asperger syndrome, pervasive developmental disorder–not otherwise specified [PDD-NOS]) in children.

Design, Setting, and Patients  The study sample of 85 176 children was derived from the population-based, prospective Norwegian Mother and Child Cohort Study (MoBa). The children were born in 2002-2008; by the end of follow-up on March 31, 2012, the age range was 3.3 through 10.2 years (mean, 6.4 years). The exposure of primary interest was use of folic acid from 4 weeks before to 8 weeks after the start of pregnancy, defined as the first day of the last menstrual period before conception. Relative risks of ASDs were estimated by odds ratios (ORs) with 95% CIs in a logistic regression analysis. Analyses were adjusted for maternal education level, year of birth, and parity.
Main Outcome Measure  Specialist-confirmed diagnosis of ASDs.
Results  At the end of follow-up, 270 children in the study sample had been diagnosed with ASDs: 114 with autistic disorder, 56 with Asperger syndrome, and 100 with PDD-NOS. In children whose mothers took folic acid, 0.10% (64/61 042) had autistic disorder, compared with 0.21% (50/24 134) in those unexposed to folic acid. The adjusted OR for autistic disorder in children of folic acid users was 0.61 (95% CI, 0.41-0.90). No association was found with Asperger syndrome or PDD-NOS, but power was limited. Similar analyses for prenatal fish oil supplements showed no such association with autistic disorder, even though fish oil use was associated with the same maternal characteristics as folic acid use.
Conclusions and Relevance  Use of prenatal folic acid supplements around the time of conception was associated with a lower risk of autistic disorder in the MoBa cohort. Although these findings cannot establish causality, they do support prenatal folic acid supplementation.

ADDENDUM: I've recently discovered that around half the USA has a genetic mutation that makes it hard for them to actually use the folic acid that is put in white flour, breakfast cereal, and most multivitamins. To bypass the problem, it's better to ingest L-methylfolate, which is exactly the form that our bodies use. On average, it is better for increasing our blood folate status and doesn't mask B-12 deficiency. And get the word out so that food producers will start putting L-methylfolate in their products instead of plain folic acid.

Notice that in the Norway study above, autism went down by half in the group that took folic acid. Maybe it would have gone away almost completely if they'd given all the mothers L-methylfolate, seeing as around half the women likely couldn't use folic acid very well.

Final Update: Here's a link to a letter to the editor that a friend and I co-authored where we discuss the possible connections between folic acid overuse, inattentive-type ADHD, and autism spectrum disorders. It was published in the International Journal of Food Sciences and Nutrition in June of 2016.