By Robert Bazell, NBC News chief science correspondent

Many studies have shown that eating fish is healthy, but mercury which comes from industrial pollution makes its way into the ocean and into the meat of fish. Adults who eat very large quantities of fish can get mercury poisoning. But the biggest concern is the effect on the brains of fetuses and young children when pregnant or nursing mothers eat fish.
 
You can read current government guidelines about seafood and mercury here.

Here are two summaries of scientific articles from a group at Harvard that illustrate both sides of this issue:

The first, a survey of more than 25,000 birth records in Denmark shows that children whose mothers eat more fish do better developmentally.

The second shows a correlation between higher mercury levels in the mother's blood and less development in the children.