What goes up as industrial pollution comes down as, among other things, methylmercury in bodies of water. When our seafood swims in that contaminated water, it turns into mercury in our food supply.
In adults, the relatively small amount of mercury found in fish isn't a big deal. Our bodies can get rid of it. But in a developing fetus, the mercury in a few weekly albacore-tuna sandwiches has the potential to damage the nervous system. But here's where it gets tricky, because avoiding all fish during pregnancy is a bad idea (see No. 6). When it comes to seafood in a pregnancy diet, it's all about selectivity.
The big predator fish typically contain the most mercury (they've been feeding on all the other, smaller mercury-contaminated fish, and they live longer). This includes tilefish, swordfish, King mackerel and shark [source: Bouchez]. It's best to avoid these fish completely.
Albacore tuna tends to be higher in mercury than canned "light tuna." If you can't live without albacore, limit consumption to 6 ounces per week (about one serving) [source: Bouchez].
For other fish, limit consumption to 12 ounces per week. Shrimp, crab, salmon, tilapia, light tuna, anchovies and catfish are good choices. This should keep the mercury level safe for a developing baby. See FDA: Mercury Levels in Commercial Fish and Shellfish for a complete list.
Since it's best to err on the side of caution, the obvious question is, why not eliminate fish from the pregnancy diet? Up next: why that would be a mistake.