Even for perfectly healthy individuals, eating raw shellfish can be dangerous. When you don’t cook food, any bacteria that were originally on the animal or plant will make its way right into your digestive system. During your pregnancy, even cooked shellfish can be harmful, so it’s important that you avoid such food as much as possible.

When most women become pregnant for the first time, they’re under the impression that they cannot eat any fish because of the high levels of mercury likely in the waters that they came from. In reality, these levels aren’t nearly as high as they used to be, and fish are perfectly fine in moderation. On the other hand, shellfish store the mercury in their bodies differently. Since their digestive systems are slower than that of a speedy tuna or shark, any mercury seeps deeply into the very same tissues that you eat. It makes sense when you consider the sedentary lifestyle of mussels or clams.

Mercury isn’t the only reason shellfish are dangerous, either. You might have heard of the “red tide” as it relates to fishing. The red tide happens when a large amount of a particular type of algae makes its way towards the shore and infects the fish and shellfish there. When shellfish become infected with the algae, they become extremely dangerous to humans. Especially when eaten raw, the algae can cause illnesses such as neurotoxic shellfish poisoning (NSP). Since pregnant women are immune-compromised, NSP is especially dangerous. When a pregnant woman becomes infected with NSP, there’s a good chance her fetus will soon be infected as well.

All in all, shellfish during your pregnancy are not safe. You should speak with your doctor about it if you’ve had any without realizing the potential risks, but it’s just one of those foods that you can’t trust. Luckily, you can still eat fish in moderation, but shellfish pose a greater risk based on the physiology of their bodily systems. When bacteria, algae, or chemicals make their way into the body and tissue of a simple shellfish, it doesn’t make its way out very easily. By the time you consume it, there’s a good chance anything that was ever in them will also be in your system. Giving up your favorite mussels might be tough for nine months, but the sacrifice will be well worth it in the end.

Source: Emily Oiken et al: Maternal Fish Intake During Pregnancy, Blood Mercury Levels, and Child Cognition at Age 3 Years in a US Cohort. American Journal of Epidemiology Volume 167 Issue 10 pp. 1171-1181 January 2008