There is a great deal of information in print and on the internet warning against consuming certain fish in pregnancy.  It is important to stay informed regarding this - too much of certain fish can increase a baby's mercury exposure, and mercury can negatively impact a baby's brain development.  On the other hand, fish are high in omega acids, which have been associated with better brain development.  The differences between omega acids found in prenatal vitamins, and that consumed from food, are very different.

 
Fresh tuna can be relatively high in mercury - it is recommended that a pregnant patient consume 6 ounces or less during pregnancy.  White albacore tuna, on the other hand, is what is found in most canned tuna.  
 
The Dietary Guidelines Advisory Committee encouraged the FDA and the EPA last year to reconsider their guidelines on consuming 6 ounces of tuna or less per week.  According to DR. Steve Abrams, medical director of the Neonatal Nutrition Program at Baylor College of Medicine, there is strong evidence that fish consumption is good for the developing baby's brain, and canned tuna can be an important source for pregnant women because it is relatively cheap, and easy to buy and prepare.  For a comparison, 6 ounces of swordfish contains 170 mcgs of mercury, canned tuna 60 mcgs, salmon 4 mcgs.
 
A wide variety of fish can be beneficial for fetal, and even neonatal brain development.  Shrimp, scallops, salmon, and tilapia and haddock( the latter two are sometimes used in fast food fish meals) are some of the lowest in mercury.  Haddock, pollock, catfish, trout, sole, and flounder are also very low.  Rest assured that white albacore tuna is safe, and that 12 ounces per week is probably safe, and beneficial for the development of the baby's brain!  Bon Appetit!