Short answer: Two main reasons, one foundational and one more recently confirmed. The foundational reason is that DHA is a direct structural building block for the fetal brain and retina, accumulating rapidly during pregnancy at a rate the fetus can’t fully produce on its own. The more recently confirmed reason, and genuinely one of the stronger evidence-backed findings in this area, is that adequate omega-3 intake measurably reduces the risk of preterm and early preterm birth, a conclusion significant enough that U.S. federal dietary guidance has now formally recognized it.
The Foundational Reason: Building the Brain and Eyes
DHA is concentrated in the fetal brain and retina at levels far higher than most other tissues in the body, and it accumulates especially rapidly during the third trimester as the visual system and brain undergo intense structural development. Because the fetus has limited capacity to produce DHA on its own, it depends on a steady maternal supply transferred through the placenta. This structural rationale is well established and forms the baseline reason omega-3 intake gets attention during pregnancy in the first place, independent of any other specific health outcome.
The Genuinely Strong, More Recently Confirmed Reason: Preterm Birth Reduction
This is where the evidence has become considerably stronger and more specific in recent years, and it’s worth giving this finding its full due. A landmark 2018 Cochrane review analyzing 70 randomized controlled trials found that increasing omega-3 intake during pregnancy produced an 11 percent reduction in all-cause preterm birth and a 42 percent reduction in early preterm birth compared with control groups, findings that held up and were reaffirmed in a 2022 update to the same review. A 42 percent reduction in early preterm birth specifically, the more severe and medically consequential category of premature delivery, is a substantial effect for a dietary intervention, and it’s backed by one of the largest and most rigorously conducted evidence reviews in this area of nutrition science.
This evidence has become significant enough that it’s now reflected in official U.S. guidance. The National Institutes of Health’s Office of Dietary Supplements updated its guidance to state plainly that research indicates long-chain omega-3s reduce the risk of preterm birth, and experts now recommend that all women of childbearing age consume at least 250 milligrams a day of combined DHA and EPA, with an additional 100 to 200 milligrams of DHA during pregnancy specifically. This kind of formal recognition from a major government health agency reflects a considerably higher evidentiary bar than general nutritional advice typically requires.
Most Pregnant Women Fall Short of These Targets
This recommendation matters in practice because it addresses a real, common gap. The same federal guidance notes that most women who are pregnant do not currently meet these omega-3 intake recommendations, which is part of why doctors and dietitians are actively raising this topic rather than assuming it’s already covered by a typical diet. Between limited fish consumption in many diets and the increased demand pregnancy places on maternal omega-3 stores as the fetus draws on them, the gap between typical intake and the recommended target is a genuinely common one.
What This Means in Practice
Put together, doctors recommend omega-3 during pregnancy because the underlying biological case for fetal brain and eye development has always been strong, and because more recent, large-scale research has added a second, independently significant reason: a measurable reduction in preterm birth risk, an outcome with major implications for infant health that goes well beyond the original developmental rationale.
- DHA is a direct structural component of the fetal brain and retina, built up rapidly during pregnancy. This foundational biological role has been recognized for decades.
- Adequate omega-3 intake has been shown to meaningfully reduce preterm and early preterm birth risk. A 42 percent reduction in early preterm birth, found in a major Cochrane review, is now reflected in official U.S. federal guidance.
- Most pregnant women don’t currently meet the recommended 250 mg/day baseline, plus the additional pregnancy-specific amount. This is a genuinely common, addressable gap rather than a rare edge case.
As with any pregnancy-related nutrition question, this is general information rather than personalized medical advice, and specific questions about omega-3 intake or supplementation during pregnancy are worth bringing directly to a doctor or midwife.
