According to a 2011 study (1), the researchers found that mothers of children with autism were less likely than those of typically developing children to report having taken prenatal vitamins during the 3 months before pregnancy or the first month of pregnancy. Greater risk was also observed for children whose mothers had other one-carbon metabolism pathway gene variants and reported no prenatal vitamin intake.

Their conclusion is that periconceptional use of prenatal vitamins may reduce the risk of having children with autism, especially for genetically susceptible mothers and children.

Reference: 
1. Schmidt RJ, Hansen RL, Hartiala J, Allayee H, Schmidt LC, Tancredi DJ, Tassone
F, Hertz-Picciotto I. Prenatal vitamins, one-carbon metabolism gene variants, and
risk for autism. Epidemiology. 2011 Jul;22(4):476-85.

While a lot of pregnant women knows the importance of increasing their intake of Omega-3, it is important that we keep ourselves informed with the various kind of seafood that can contain high levels of mercury. High levels of mercury is not good for the nerves, brain and heart and might cause harm in the fetus.

Some types of fish contain much more mercury, on average, than others. However, mercury levels can vary between populations of the same species depending on where they reside and migrate. 

Reference:
1. http://www.bbc.co.uk/