In the past, women were warned against eating more than 12 ounces of fish per week during their pregnancy due to the high levels of mercury found in fish. It was believed that high levels of mercury would harm an unborn baby’s developing nervous system, when in fact; there are no recorded cases of fetal mercury toxicity.
More recent research has shown that there are many benefits of consuming fish during a pregnancy. And, these benefits far outweigh the small risk associated with Mercury levels.
The primary benefit of consuming fish during pregnancy comes from the high levels of Omega-3 fatty acids. Fatty acids are especially beneficial during the second and third trimesters of pregnancy, when your baby’s nervous system is forming.
The Food and Drug Administration still recommends that a pregnant woman limit fish consumption to 12 ounces of fish per week during pregnancy, but further studies are underway and this recommendation will probably change. 
If you are concerned about the levels of mercury in fish, discuss it with your doctor. They can recommend what types of fish contain lower amounts of mercury and how much fish you should consume.