Excessive Folate May Increase ASD Risk

Doctors have long told pregnant women to get enough of the nutrient folate to ensure proper brain development in their babies, spurring widespread use of prenatal vitamins as well as fortified foods and a decrease in the incidence of certain birth defects.

New research, however, suggests there can be too much of a good thing.
A small study from the Johns Hopkins Bloomberg School of Public Health found that excessive folate levels as well as high levels of vitamin B12 in mothers appear to raise the risk of autism spectrum disorders in children.

Source

Another opinion: 

Concerns About Folate Causing Autism Are Premature

Meanwhile in 2013 - "A recent study claims that women who take folic acid supplements before conception are at a lower risk of having an autistic child."