Sadly for those people who love to eat fish food, it is not quite safe to eat them in pregnancy. The reason for this is that sea water is contaminated with mercury now more than ever, due to military and chemical practices. For hundreds of years, man has used rivers and the sea to dump their waste. And of course there has been many spills of fuels and chemicals in the sea. Due to this the fish are contaminated, and it is of special concern to small children and pregnant women.
It is recommended that you limit your fish intake to once every month for certain fish foods. For tuna for example, you are allowed a shorter span, every couple of weeks should be safe.
The fact about mercury harming a pregnant woman increases with many of her individual situations. Dental work used to be composed of mercury, before it was found to be harmful to human health, specially to woman who might become pregnant.
Lots of dental fillings done before 2003 or so, contained a high amount of mercury which is shed as the person eats or sleeps. The biggest concern of this is the fact that mercury stays in your blood for months. So the mercury accumulated by the dental fillings, mixed with mercury from fish and other sources would lead a pregnant woman to have too much.
Mercury affects the unborn baby's brain cells, which is irreversible. This is commonly known as brain damage.
To read more about it, please take a look at these sources: