Probably not so great even if you're not pregnant...Mercury in the upper layers of the ocean has more than tripled since industrial revolution, mostly thanks to the burning of fossil fuels like coal. Marine predators at the top of the food chain eat lots and lots of smaller creatures which themselves have mercury body burdens, creating an accumulation of mercury in their tissues. Since mercury is fat soluble and not easily eliminated, fish-tissue concentrations increase over time... Until the day they end up on the diner plate of someone at the very top of the food chain and transfer that neurotoxic mercury to them.
This is why the FDA has recently issue new recommendations for pregnant women and parents to help them reduce their intake of mercury by either reducing their fish consumption, or by switching to species that are less likely to be contaminated.
Substitute safe, low-mercury alternatives
As you can see, various varieties of tuna contain on average a fairly high amount of mercury. Other species of fish also rank very high, but they tend to be a lot less common and be eaten less often by most people. Note at the bottom the varieties that are not recommended in any quantity to pregnant women, women who are breastfeeding, or young children. Not that anyone should be eating shark in the first place...
The FDA only recommends that vulnerable people limit their fish eating to 2 or 3 servings of fish a week and avoid the 4 types of fish mentioned above.
Consumer Reports tested a bunch of tuna samples and decided to go farther than the FDA: As a precaution, pregnant women should avoid all tuna, period.
This is because every sample they tested contained mercury, and research is showing that even small quantities can have negative impacts, especially on fetuses and young children.
What if you still want to eat fish? Consumer Reports says that low-mercury alternatives, like shrimp (not exactly fish, but a popular seafood) and Alaskan salmon should be preferred because they contain much lower amount of mercury, as well as other toxins like PCBs.
And if you want to eat fish for the omega 3 fatty acids, you should probably look at supplements which come from sources that are lower on the food chain and thus very likely to be safe.
Just to confuse things further: DNA testing shows 59% of fish sold as 'tuna' in U.S. is something else.