Fish and Mercury During Pregnancy: How Much Is Safe

Fish is genuinely one of the best food groups for pregnancy — rich in omega-3 fatty acids, protein, and iodine — but it’s also the one food category where portion limits and specific avoid-lists matter more than almost anywhere else in a pregnancy diet. The reason is mercury, and understanding how it works makes the guidance far less confusing.

Why Mercury Matters

Mercury occurs naturally in the ocean and accumulates in fish tissue over their lifetime. Larger, longer-living predatory fish — the ones at the top of the ocean food chain — accumulate the most, because they’ve eaten many smaller fish that each contained a small amount of mercury. High mercury exposure during pregnancy can affect a developing baby’s nervous system, which is why pregnant women are advised to limit or avoid the highest-mercury fish specifically, rather than avoiding fish altogether.

This is an important distinction: the answer to mercury risk is not “eat less fish” as a general rule. Most fish are low in mercury, genuinely good for you, and actively recommended in pregnancy. The guidance is about which specific fish to limit, not fish as a category.

Fish to Avoid Entirely

A small number of large predatory fish carry high enough mercury levels that the NHS recommends avoiding them completely during pregnancy:

• Shark
• Swordfish
• Marlin

These three are singled out specifically because of consistently high mercury readings, and the recommendation is avoidance rather than a portion limit.

Fish to Limit

Tuna sits in its own category: safe, but limited due to mercury content that’s meaningfully higher than most other fish, though not high enough to warrant the full avoidance applied to shark, swordfish, and marlin.

• Fresh or frozen tuna steaks — no more than two steaks per week
• Tinned tuna — no more than four medium tins per week

If you eat tinned tuna, it doesn’t count toward your general “two portions of fish per week” allowance in the same way — it has its own separate limit specifically because of the mercury consideration.

The General Fish Guidance

For fish that don’t carry a specific mercury warning, the standard NHS recommendation is:

• Up to two portions of fish per week, of which
• One portion should be oily fish — such as salmon, mackerel, sardines, or trout — for the omega-3 benefit

Oily fish carries its own separate portion limit (rather than an avoid-list) because of a different consideration: environmental pollutants that can build up in fattier fish tissue over time. Two portions a week keeps this well within safe limits while still delivering strong omega-3 benefits.

Fish You Can Eat Freely

The majority of commonly eaten fish don’t carry any specific mercury or pollutant warning beyond the general two-portions guidance, including:

• Cod and other white fish
• Salmon (fresh, tinned, or hot-smoked)
• Prawns and most shellfish (cooked)
• Trout
• Haddock
• Plaice

A Note on Raw Fish and Sushi

Mercury is a separate issue from the raw fish safety concerns covered elsewhere on this site — cooked sushi rolls are safe with no special mercury consideration beyond the usual guidance, while raw fish carries its own separate set of considerations around bacteria and parasites rather than mercury specifically.

Quick Reference

Category	Guidance
Shark, swordfish, marlin	Avoid entirely
Tuna (fresh)	Max 2 steaks per week
Tuna (tinned)	Max 4 medium tins per week
Oily fish (salmon, mackerel, sardines)	Max 1 portion per week as part of your 2-portion total
Other fish (cod, prawns, trout)	Up to 2 portions per week, no specific limit beyond this

Getting this balance right means you can enjoy the genuine nutritional benefits fish offers during pregnancy — supporting your baby’s brain and eye development in particular — while keeping mercury exposure well within safe limits.