Women who consume the recommended daily dosage of folic acid (600 mcg) during the first month of pregnancy may have a reduced risk of having a child with autism.  The authors recommend that not only should women who have already conceived take folate,  those who are attempting to conceive should also consider taking folic acid supplements.

Folic Acid Intake During Early Pregnancy Associated With Reduced Risk of Autism in Offspring (Science Daily)

Since about half of all pregnancies are unplanned, should all reproductive aged women take these supplements?