Fish oil is generally safe during pregnancy and provides omega-3 fatty acids that support fetal development. The American College of Obstetricians and Gynecologists lists omega-3s among the key nutrients needed during pregnancy, and the FAO/WHO recommends pregnant women consume 200 to 300 mg of DHA per day, up to 1,000 mg. The important distinction is between standard fish oil (safe) and cod liver oil (potentially risky), along with paying attention to dose and product quality.
Why Omega-3s Matter During Pregnancy
The two omega-3 fatty acids in fish oil, DHA and EPA, play specific roles in fetal growth. DHA is particularly important for myelination, the process that insulates nerve fibers in your baby’s developing brain, and for vision development during the period around birth. These aren’t nutrients your body manufactures efficiently on its own, so they need to come from food or supplements.
A systematic review of eight randomized trials found that DHA supplementation during pregnancy (ranging from 200 to 2,200 mg per day) improved measures of cognitive development in infants by 6% to 11% in five of those studies. The evidence is promising but not airtight: all eight studies also reported at least one outcome that didn’t reach statistical significance, and evidence for benefits to language, motor skills, social-emotional development, and vision was inconsistent. Still, the overall picture supports omega-3 intake during pregnancy as beneficial for early brain development.
Preterm Birth Risk Reduction
One of the strongest arguments for fish oil during pregnancy is its connection to carrying longer. A 2018 Cochrane Review covering 27 randomized trials found that omega-3 supplementation reduced the risk of premature rupture of membranes by 59% and preterm prelabor rupture of membranes by 47%. In the more recent ADORE trial, participants who started with low DHA levels and stuck with a high-dose regimen saw a 58% reduction in preterm birth and a 65% reduction in early preterm birth (before 34 weeks). Even across all adherent participants regardless of baseline levels, preterm birth dropped by 50%.
These numbers are striking, though they come with a caveat: the benefits were most dramatic in women who had low omega-3 levels to begin with. If you already eat fatty fish regularly, supplementation may offer a smaller advantage.
Fish Oil vs. Cod Liver Oil
This is the single most important safety distinction. Standard fish oil, made from the body of the fish, contains omega-3s with minimal vitamin A. Cod liver oil, made from the liver, contains high levels of preformed vitamin A (retinol), which can harm a developing baby in excess.
The UK government explicitly recommends that pregnant women avoid cod liver oil and any supplements containing vitamin A unless directed by a doctor. Excess retinol during pregnancy is linked to birth defects, particularly in the first trimester. If you’re shopping for a fish oil supplement, check the label carefully. Look for products labeled as fish body oil and confirm they don’t contain added vitamin A or vitamin D from liver sources.
Mercury and Contaminant Concerns
Mercury is a legitimate worry with seafood during pregnancy, but fish oil supplements pose very little risk on this front. Most commercial fish oils are purified through molecular distillation, a process that removes mercury, lead, and other environmental contaminants to levels well below safety thresholds. This makes supplements a practical workaround for women who want omega-3 benefits without the mercury exposure that comes with eating certain fish like swordfish, king mackerel, or tilefish.
One thing worth noting: the supplement industry isn’t regulated by the FDA the same way medications are, so product quality varies. Look for brands that carry third-party testing seals (USP, NSF, or IFOS are common ones). These organizations verify that what’s on the label matches what’s in the capsule and that contaminant levels are within safe limits.
Oxidation and Freshness
Rancid fish oil is a less obvious but real concern. Omega-3 fatty acids are chemically fragile and oxidize easily when exposed to heat, light, or air. Multiple reports have found commercially sold fish oil products that were already oxidized before reaching consumers. The health effects of consuming oxidized fish oil aren’t fully understood yet, but oxidized fats generally promote inflammation rather than reducing it, which defeats the purpose of taking the supplement.
You can do a simple quality check at home. Break open a capsule and smell or taste the oil. Fresh fish oil has a mild, slightly oceanic smell. If it smells strongly fishy, bitter, or paint-like, it’s likely rancid. Store your supplements in a cool, dark place (the refrigerator works well) and check the expiration date before buying.
Bleeding Risk Near Delivery
Fish oil has mild blood-thinning properties, which raises a reasonable question about whether it could cause problems during labor or a C-section. A large meta-analysis published in the Journal of the American Heart Association found that high-dose purified EPA (one specific type of omega-3, taken at prescription-level doses for heart conditions) increased the relative risk of bleeding by 50%. That sounds alarming, but the absolute increase was only 0.6%, translating to one extra bleeding event for every 166 people treated. Importantly, the review found no increased risk of serious bleeding events like hemorrhagic stroke, intracranial bleeding, or gastrointestinal bleeding.
These findings came from studies using high-dose, purified EPA formulations typically prescribed for cardiovascular disease, not standard prenatal fish oil doses. At the 200 to 1,000 mg range recommended during pregnancy, the bleeding risk is minimal. That said, some providers suggest stopping fish oil supplements two to three weeks before a scheduled C-section or if you’re on blood-thinning medications, so it’s worth mentioning your supplement use during prenatal visits.
How Much to Take and What to Look For
The FAO/WHO recommends a minimum of 200 to 300 mg of DHA daily during pregnancy, with an upper range of 1,000 mg. Many prenatal vitamins include some DHA, but the amount varies widely. Check your prenatal label first, then supplement the difference if needed. A standalone fish oil capsule typically contains 200 to 500 mg of DHA per serving.
When reading labels, pay attention to the DHA and EPA amounts specifically, not just the total “fish oil” content. A capsule might contain 1,000 mg of fish oil but only 300 mg of actual omega-3s. The rest is other fats that don’t provide the same benefits. You can also get DHA from algae-based supplements, which are a good option if you’re vegetarian or concerned about fish-sourced products. Algae oil provides DHA directly (it’s where fish get their DHA in the first place) without the contaminant concerns associated with the marine food chain.