For a number of years pregnant women or those planning pregnancy have been warned by numerous government agencies and medical associations to keep seafood intake to a minimum to avoid mercury which can affect brain development and is found in many species of fish and other sea animals.
However, the benefits of omega 3 may outweigh the risks in pregnancy, according to a paper published in the Lancet last month by Professor Joseph Hibbeln. A survey of 11,875 pregnant women found that those who ate less than 340 grams (3 servings) of seafood per week in pregnancy had more adverse outcomes. Beneficial effects on child development were recorded in those pregnancies where the mother's seafood intake was greater than 340 g per week, suggesting that advice to limit seafood consumption could actually be detrimental.
Mothers who ate more than 3 servings of seafood per week, had children who:
were more advanced in developmental tests measuring fine motor, communication and social skills as toddlers;
had more positive social behaviours; and were less likely to have low verbal IQ scores at the age of 8.
Conversely, the less fish the mothers ate the more likely the children were to perform poorly in behaviour and cognitive skills. For example, those children whose mothers had eaten no fish were:
28% more likely to have poor communication skills at 18 months
35% more likely to have poor fine motor coordination at age three and a half
44% more likely to have poor social behaviour at age seven
48% more likely to have a relatively low verbal IQ at age 8
when compared with children of women who ate more than 3 servings per week.

The Lancet, 2007; 369:578-585
For the abstract, visit:
http://www.thelancet.com/journals/lancet/article/PIIS0140673607602773/abstract

Comment:
Larger fish like tuna tend to be higher in mercury so don't eat tuna more than once a week and since canned tuna has much less of the omega-3's left after the canning process, stick with fresh tuna. Marlin and swordfish should not be eaten more than once a month. Generally speaking, the larger the fish the more the mercury, so eat salmon, small mackerel or sardines instead. Farmed salmon may have significantly lower amounts of omega-3 compared to wild salmon because the amount of omega-3 in the fish depends to a large extent on the quality of its diet.

Ref: http://www.foodforthebrain.org