I've heard people say that pregnant women shouldn't eat a lot of fish because of the mercury content. What's so bad about mercury and why is it in fish?

Although fish contain high-quality protein and other essential nutrients, are low in saturated fat, and contain omega-3 fatty acids, nearly all fish and shellfish contain traces of mercury.

Mercury occurs naturally in the environment, and can also be released into the air from pollution. The mercury falls from the air and accumulates in oceans, lakes, and rivers. Fish absorb the mercury since they live and feed in these waters, and the mercury builds up their bodies. That's why larger fish are the ones to watch out for when it comes to mercury, because they've had more time to accumulate it.

Some fish and shellfish contain higher levels of mercury that may harm an unborn baby or young child's developing nervous system. That's why nursing mothers, women who are pregnant, and those who are trying to become pregnant are advised to watch their mercury intake.

For everyone, elevated levels of mercury can lead to nervous system problems with brain or nerve damage. Mercury levels can be measured by blood tests, or tests using hair.

If you regularly eat types of fish that are high in mercury, it can accumulate in your blood stream over time too. The body naturally removes it, but it may take over a year for levels to drop back to normal. That's why it's important for women who are trying to get pregnant to think about their mercury intake beforehand.

Want to hear about some guidelines from the EPA? Then read more

1. Do not eat Shark, Swordfish, King Mackerel, or Tilefish because they contain high levels of mercury.
2. You can eat up to 2 meals (total of 12 oz) of fish a week.
3. 5 of the most commonly eaten fish that are low in mercury are shrimp, canned light tuna, salmon, pollock, and catfish.
4. Albacore ("white") tuna has more mercury than canned light tuna. So, when choosing your 2 meals of fish and shellfish, you may eat up to 6 oz (1 average meal) of albacore tuna per week.
5. Check local advisories about the safety of fish caught in lakes, oceans, and rivers near you. If no advice is available, only eat 1 meal (6 oz) a week of fish you catch, and no other fish that week.

Check out this poster. It's a great visual for which fish are low in mercury, and which ones to watch out for.