Can I eat tuna while pregnant?

The verdict

Tuna is not banned anywhere. It is limited everywhere, and the limits are strikingly different, which is why this question generates so much confusion.

The NHS says you should not eat more than four cans of tuna a week or more than two tuna steaks a week. Its figures are based on a medium can with a drained weight of around 140g and a 140g cooked steak.

Ireland's HSE sets roughly half that: tuna should be limited to one 150g cooked weight fresh tuna steak, or two 240g cans with a 140g drained weight, per week.

The FDA does not count cans. It sorts tuna by species. Canned light tuna, which includes skipjack, is a Best Choice, and pregnant women can eat two to three servings a week of Best Choices, a serving being four ounces. Albacore or white tuna, canned and fresh or frozen, and yellowfin tuna are Good Choices, of which the advice is one serving a week. Bigeye tuna is on the Choices to Avoid list for highest mercury levels.

Food Standards Australia New Zealand does not restrict tuna at all in its mercury advice. The species it limits are orange roughy and catfish, at one 150g portion a week and no other fish that week, and shark, also called flake, and billfish, meaning swordfish, broadbill and marlin, at one portion a fortnight for women who are pregnant or planning pregnancy.

Why the numbers differ so much

The hazard is the same everywhere: methylmercury, which accumulates up the food chain and crosses the placenta. FSANZ explains the mechanism plainly. Mercury occurs naturally in the environment and builds up in fish over time; the amount depends on the age of the fish, the environment it lives in and what it eats; and big, long-lived predatory fish carry more than small, young ones. High levels can damage the nervous system, and unborn babies are especially vulnerable because they are going through rapid growth and development.

What differs is which tuna each country's population actually eats. Tuna is not one fish. Skipjack, the small fast-growing species in most cans of light tuna, carries far less mercury than bigeye, a large deep-water predator sold as sushi-grade steak. Albacore sits in between.

British advice bundles all canned tuna together and sets a number of cans, which is simple to follow and necessarily conservative for skipjack and generous for albacore. American advice splits by species, which is more precise and requires you to read the tin. Irish advice takes the British approach and sets a tighter number. Australian advice concludes that the tuna sold there does not need a specific limit and puts the restrictions on genuinely high-mercury species instead.

None of these is wrong. They are different regulatory judgements about the same underlying data, applied to different national fish supplies.

Tuna is not an oily fish

This trips people up constantly. The NHS is explicit that fresh and canned tuna do not count as oily fish, and that the two-portions-a-week limit on oily fish is separate. If you have had a portion of tuna during the week you can still have up to two portions of oily fish, which for a pregnant woman means salmon, sardines, mackerel, trout, herring or pilchards.

The oily fish limit exists for a different reason: pollutants that build up in the body over time and may affect the future development of a baby in the womb. The tuna limit is about mercury.

What counts as a can, a steak and a serving

The UK figure is per medium can with about 140g drained weight. A large catering tin is more than one can. A tuna sandwich or a jacket potato filling made from half a can is half a can.

The Irish figure is stated with the tin size on it: two 240g cans, meaning a 140g drained weight each, per week, or one 150g cooked-weight fresh steak.

The FDA's serving for pregnancy is four ounces, roughly 113g, and it suggests using the palm of your hand as a guide. Its overall recommendation is that those who are pregnant or breastfeeding eat between eight and twelve ounces per week of a variety of seafood from lower-mercury choices.

All three sets of numbers are weekly, not daily, and none of them is a threshold at which harm begins. They are intake ceilings built with a margin below the level of concern.

Sushi-grade tuna, ahi and raw preparations

Raw tuna raises a second, separate issue that has nothing to do with mercury. The NHS lists raw fish and raw shellfish among foods to avoid, and sushi made with raw fish, even if it has been previously frozen. The HSE lists raw or smoked fish, including sushi, on its avoid list. ACOG advises avoiding all raw and undercooked seafood and says not to eat sushi made with raw fish, while cooked sushi is safe.

Ahi tuna is usually yellowfin or bigeye, both of which sit higher on the FDA's mercury scale, and it is usually served rare or raw. A seared tuna steak with a raw centre is therefore doubly restricted: species and preparation. If you want tuna as a steak, cook it through and choose a species that is not bigeye.

Canned tuna, and what it is good for

Within the limit, canned tuna is genuinely useful in pregnancy. It is shelf-stable, needs no cooking, and is a straightforward source of protein and, in the case of tuna canned in oil, some fat. Because it is heat-processed in the can it does not carry the listeria concern that applies to chilled ready-to-eat seafood.

Tuna in a pre-packed sandwich is a different food. The NHS groups pre-packaged sandwiches and salads containing fish and shellfish among the foods to be careful with, to be kept chilled and eaten before the use-by date, because ready-to-eat foods can carry listeria. Tuna mayonnaise made at home from a can and shop-bought mayonnaise avoids that entirely.

If you have gone over

Going over the weekly limit once is not an emergency and does not require any test or intervention. Mercury accumulates and clears slowly, and the guidance is built around long-run average intake, not around single meals.

The NHS's general position on foods that should have been limited or avoided is that a small amount, if you did not become ill, is unlikely to affect your baby. The sensible response to a heavy tuna week is to eat something else next week rather than to worry about the one that has passed. If you have been eating tuna daily for a sustained period, mention it at your next appointment so it can be looked at properly.

Where guidance genuinely differs

On the direction, nowhere: every authority wants pregnant women to eat fish, and says so. The FDA's advice opens by stating that fish provide key nutrients that support a child's brain development, including omega-3 fats, iron, iodine and choline. FSANZ makes the same case. The NHS recommends at least two portions of fish a week, including one of oily fish.

On tuna specifically, the four positions are: the UK counts cans and allows four; Ireland counts cans and allows two; the US counts species and portions and rules out bigeye; Australia and New Zealand do not limit tuna but restrict shark, billfish, orange roughy and catfish instead.

If you want a single number that satisfies all of them, take the Irish one, avoid bigeye, and you are inside every published limit on this page.