Fish and Mercury in Pregnancy: Which Fish, How Much, How Often

Fish is the food where pregnancy advice most often gets read backwards. People hear "mercury" and stop eating fish altogether, which is the opposite of what the guidance says. The avoid list is five species long. Everything else is a portion limit at worst, and most of it is actively recommended.

The two rules people confuse

There are two separate things going on with fish in pregnancy, and almost all the confusion comes from mashing them together.

Rule one is mercury, and it applies to a specific short list of species. Large predatory fish live a long time and eat other fish, so methylmercury accumulates in their flesh over years. That is a genuine reason to avoid them, and it's why they are excluded rather than limited.

Rule two is portions, and it applies to oily fish that are good for you. The cap exists because of low-level pollutants that build up slowly, not because any single meal is a problem. Two portions a week is the ceiling, and one to two is the recommendation.

Cooking is a separate issue, and it has nothing to do with mercury. That part comes later.

The avoid list

Don't eat the following fish during pregnancy:

• Swordfish
• Shark
• Marlin
• King mackerel
• Bigeye tuna

FDA and EPA guidance also includes orange roughy and tilefish from the Gulf of Mexico on the avoid list. UK guidance tends to lead with swordfish, shark and marlin, while tuna and oily fish are handled through limits rather than a full ban unless you are talking about bigeye tuna specifically.

One point is worth clearing up because the names are easy to muddle. King mackerel is not the same fish as the Atlantic mackerel sold in most UK supermarkets. Atlantic mackerel is a smaller oily fish, lower in mercury and one of the more useful fish you can eat in pregnancy. So if a fishmonger in the UK sells you "mackerel", it's almost certainly the ordinary oily fish, not the high-mercury one on the US avoid list.

The weekly limits

Per NHS guidance:

• Oily fish: up to two portions a week. Salmon, trout, sardines, mackerel, herring, pilchards and sprats carry the omega-3 DHA that is hardest to get anywhere else. One to two portions is the target rather than the tolerance.
• Canned tuna: up to four cans a week. Canned tuna is usually skipjack, a smaller and shorter-lived fish, which is why you can eat more of it.
• Fresh or frozen tuna steaks: up to two a week. This is the one fish where the same species has two different rules depending on how it was sold.
• White fish: no specific limit. Cod, haddock, pollock, plaice, sea bass and hake are low in mercury, a useful source of iodine and protein, and a reasonable default when you want fish without doing arithmetic.
• Shellfish: no limit, but cooked. Shrimp, mussels, scallops and crab are all fine when fully cooked; don't eat them raw.

Why the advice is to eat more, not less

Oily fish is one of the main food sources of DHA, the long-chain omega-3 fat that builds up in the baby's brain and eyes during pregnancy. That matters throughout pregnancy and becomes even more relevant later on. There is also moderate evidence from Cochrane that extra omega-3 long-chain fatty acids in pregnancy may reduce preterm birth and early preterm birth, so this is not just a wellness talking point repeated until it sounds true.

White fish matters for a different reason. It helps with iodine, which your body uses to make thyroid hormones, and iodine needs rise in pregnancy and then stay high while breastfeeding. Alongside dairy foods and eggs, fish is one of the more dependable ways to get it.

Raw, cured and smoked

Raw fish is off the menu. That includes sushi and sashimi made with raw fish, ceviche and fish that has been cured rather than cooked. The concern here is food poisoning and infection risk, not mercury.

Cold-smoked fish is included in that caution. This is the silky sliced smoked salmon most people picture first. It's cured rather than cooked, so heat it until steaming hot before you eat it. On a hot bagel or stirred through pasta at the end, it's fine. Cold on a blini, it's not.

Hot-smoked fish is a different product. Hot-smoked salmon, mackerel and trout are cooked during smoking, they flake rather than slice and they are not restricted in the same way. That distinction is worth learning because it puts a very useful food back within reach, especially on days when proper cooking feels like too much effort.

Sushi made with cooked fillings or vegetables is fine. Cooked shrimp, imitation crab, avocado, cucumber and similar fillings are all great options.

A practical week

If rules are easier to follow when they look like real meals, here's what a week can look like without going near any limit.

Lunch can be a can of tuna or a can of sardines.

For dinner, you can have one oily fish dinner, such as teriyaki-style salmon rice bowls or miso salmon with pickled cucumber. Or one white fish dinner, such as garlic butter cod.

That keeps you within the oily fish limit, well within the tuna limit and nowhere near the avoid list.

If you've eaten something on the list

Mercury builds up over time, not from one meal, so one portion of swordfish before you knew you were pregnant is not the emergency it can feel like in the moment. There's nothing useful to do after the fact. Mention it at your next doctor's appointment, and then let it go.

If what you ate was raw fish and you later develop a fever, flu-like symptoms, diarrhea or vomiting, call your midwife or doctor. Those symptoms are the part to act on, not the meal itself.

Where to go next

• Foods to avoid in pregnancy, for the full list beyond fish.
• Omega-3 rich recipes and iodine-rich recipes, the two nutrients fish is carrying.
• Third trimester recipes, for when DHA matters most and eating is hardest.

Sources & further reading

• FDA/EPA: Advice about eating fish
• NHS: Foods to avoid in pregnancy
• ACOG: Healthy eating during pregnancy
• Cochrane: Omega-3 fatty acid addition during pregnancy
• Food Standards Agency: Cooking your food