Fish oil is generally considered safe in pregnancy, and its main omega-3 fats (DHA and EPA) are actively encouraged for the baby's brain and eye development. Health authorities including the NHS and ACOG support omega-3 intake during pregnancy. The main caution is choosing the right product: standard fish oil is fine, but avoid cod liver oil and other liver-based oils, which are high in vitamin A.
Fish oil is a supplement rich in long-chain omega-3 fatty acids, mainly DHA and EPA. These are the same fats found in oily fish such as salmon, sardines and mackerel. The NHS and dietary guidelines recommend oily fish in pregnancy precisely because DHA supports the baby's developing brain, nerves and eyes. A supplement is one way to reach recommended omega-3 intake if you eat little fish.
Major bodies including ACOG and the NHS support omega-3 (DHA/EPA) intake during pregnancy, and DHA is a common ingredient in prenatal supplements. Reviews cited by these authorities have not linked purified fish oil to harm, and some suggest a modest reduction in early preterm birth. Fish oil is not the same as cod liver oil: the NHS specifically advises against cod liver oil and other liver oils in pregnancy because of their high vitamin A (retinol) content, which can harm the baby.
Pick a supplement labelled fish oil or omega-3 (DHA/EPA), not cod liver oil, halibut liver oil or any liver oil, to avoid excess vitamin A. Choose products purified to remove mercury and other contaminants, which the FDA flags as a concern with certain fish. Omega-3 can have a mild blood-thinning effect at high doses, so mention it to your provider if you take blood thinners or are near your due date. Getting omega-3 from oily fish (up to about two portions a week per NHS guidance) is also a good option.