Contrary to some old wives’ tales, fish isn’t just safe to eat during pregnancy, it’s actually really great for you. Fish is rich in protein and omega-3 (fatty acid), which is good for your heart and your baby’s brain development. That’s why the American College of Obstetricians and Gynecologists (ACOG) recommends pregnant women eat 2 to 3 servings a week of good-for-you fish, where one serving is equal to about the size of the palm of your hand. But not all types of fish are equally beneficial, and pregnant women should still avoid one fish-rich dish in particular: uncooked sushi.

While some types of sushi are made using cooked fish or shellfish, most kinds of this Japanese dish involve raw versions of the meat. Cooked or not, however, sushi still has all the same great nutritional benefits of most fish served in other forms, but in many cases it also carries some additional risks — most notably, heavy metals and food poisoning.

Mercury

For decades, many pregnant women were warned against eating fish because of concerns over mercury, a heavy metal that can build up in the body and damage the brain. Mercury is a naturally occurring element that exists all over the world, including in our oceans. Tiny animals take it in from the environment, and small fish eat those animals along with all the mercury inside them.

While the trace amounts in small fish don’t have much of an impact, the effect is amplified as you move up the food chain. So when we eat some big fish, we take in the mercury they’ve eaten, too. If we consume too much in too short of time, it can be dangerous, especially for pregnant women.

Not all fish is full of mercury. In fact, most fish sold in U.S. grocery stores — including canned light tuna, oysters, and salmon — don’t have much mercury at all. Pregnant women can (and should!) safely eat 2 to 3 servings of those kinds of fish a week. But some fish popular for sushi — like albacore or yellowfin tuna, snapper, and mahi mahi — have slightly higher levels of mercury and so shouldn’t be eaten as frequently (only one serving per week is recommended), or, in the cases of bigeye tuna or marlin, avoided entirely.

Listeria

While mercury should be on every pregnant woman’s radar, food poisoning is another reason to steer clear of uncooked sushi. When raw meat and seafood aren’t prepared properly, they can become contaminated with harmful microorganisms. Lots of different viruses and bacteria can contaminate food and make us sick, but a type of bacteria called listeria is the one many providers are concerned about when they talk about avoiding sushi.

Pregnant women are 20 times more likely to get sick with listeria than the general population. That’s because during pregnancy, our ability to fight diseases drops. It’s nature’s way of stopping the body from rejecting the baby. If pregnant women become infected, it can increase the risk of miscarriage, stillbirth, or early labor, not to mention lifelong consequences for the child. Given fish is widely available in cooked form — where these bacteria and viruses are killed off during the heating process — eating raw fish like in sushi is just not worth the risk.

That being said, forms of sushi that use cooked fish — like eel or shrimp tempura — can be safe and healthy for pregnant women. But if you aren’t sure what sushi items are raw or cooked, always ask your server.