NHS advice
If you are pregnant, or trying to become pregnant, you should not take fish oil supplements, particularly those that contain fish liver oil. This is because fish liver oil supplements, such as cod liver oil, contain high levels of vitamin A, which can be harmful to a developing baby.
You may have heard that the omega-3 fatty acids contained in fish oil supplements can improve a baby's cognitive development (ability to learn). However, there is currently not enough medical evidence to support this claim and more research needs to be carried out before it can be proven.
This is not to say that you should not eat fish while you are pregnant, as it is an excellent source of protein, vitamins and minerals, including omega-3. The Food Standards Agency (FSA) advises that pregnant women who eat a balanced diet do not need to take fish oil supplements because eating a range of fresh fish offers more nutrients than taking supplements alone.
Eating fish during pregnancy
Like everyone else, if you're pregnant, or trying for a baby, you should aim to eat two servings of fish a week, including one portion of oily fish, such as mackerel, or salmon.
You should avoid eating more than two portions of oily fish a week because it may contain pollutants called dioxins and PCBs (polychlorinated biphenyls). These pollutants will not cause any harm to you, or your baby, if you stick to the recommended intake of oily fish, although eating more than this can cause them to build up in your body over time.
Fresh tuna also counts as oily fish. However, you should not choose it as your weekly portion of oily fish if you are also eating canned tuna (which does not count as oily fish).
Limit your weekly intake of tuna to two fresh tuna steaks (weighing about 140g when cooked, or 170g raw) or four medium-sized cans of tuna (with a drained weight of about 140g a can).
You should also avoid eating shark, swordfish, and marlin because these fish can contain high levels of mercury.
Finally, you may also want to avoid eating raw shellfish while you are pregnant. Although shellfish will not harm your baby, there is a slight risk that it may cause food poisoning, which can be harder to deal with during pregnancy.