The United Stated Food & Drug administration has created a chart to help pregnant women in regard to safely eating seafood. Mercury is a serious issue in prenatal care and there are several species of fish known to contain much more mercury than others. Many expectant mothers choose to forgo seafood altogether just to play it safe, I know my wife did for the most part. By choosing their seafood wisely however, moms-to-be should have no problem enjoying fish during their pregnancy, even on a weekly basis.
As a general rule, the fish higher up the food chain, like shark and swordfish contain the most mercury. Tilefish and king mackerel are also to be avoided during pregnancy. However a nice salmon dinner once a week is not only allowed, it a good way to help development before birth. And when the newborn has arrived, a diet rich in seafood is beneficial for breastfeeding. Take a look at the chart below for guidelines on safe seafood consumption.
Source: US FDA