A Gene Variant Linked to Preterm Birth Risk May Help Guide Omega-3 Dosing in Pregnancy

Preterm birth remains one of the toughest problems in prenatal care, and rates in the United States haven't budged much in recent years. Omega-3 fatty acids, particularly DHA, have long been studied as a possible way to extend pregnancies closer to term, but results across trials have been inconsistent. A new analysis published in PLOS Medicine offers one explanation for that inconsistency: genetics may determine how much DHA someone actually needs.

A Gene Variant That Shifts Fatty Acid Balance

The study focuses on a specific genetic variant called the FADS Indel, found in a gene that controls how the body processes fatty acids. People who carry the "insertion," or I, version of this variant tend to produce more arachidonic acid, a fatty acid that feeds into inflammatory compounds called prostaglandins — some of which are known to help trigger labor. DHA, an omega-3 fat found in fish and prenatal supplements, works against that pathway. The researchers reasoned that people with the I allele might need more DHA to counteract this effect, while those without it (carrying two copies of the "deletion," or D, version) might not.

Notably, this isn't a rare or obscure variant — its frequency differs by ancestry, and it's more common among Black women, a group that also experiences higher rates of preterm birth in the U.S.

Revisiting a Completed Trial Through a Genetic Lens

To test this idea, the research team went back to data from the ADORE trial, a study that had already randomly assigned pregnant participants to receive either a standard 200 mg daily dose of DHA or a higher 1,000 mg dose. Of the original participants, 761 had also consented to genetic testing, allowing researchers to sort them by FADS genotype and look at how birth outcomes varied with dose.

Because this genetic breakdown wasn't the trial's original purpose, the findings are considered a secondary, after-the-fact analysis rather than a direct test of a planned hypothesis — a distinction that matters for how much weight to give the results.

What the Numbers Showed

Among women carrying at least one I allele, early preterm birth occurred in about 4.3% of those on the lower DHA dose, compared with just 1.1% of those on the higher dose. That's a notable difference, and the statistical analysis suggested a high probability that the dose itself was driving it. Among women with two D alleles, early preterm birth was already uncommon at the lower dose, and the higher dose didn't appear to help this group in the same way.

Overall preterm birth followed a similar pattern, with higher DHA dosing associated with lower rates in I-allele carriers specifically. Interestingly, the reverse signal showed up on the other end of pregnancy length: women with the D/D genotype who received high-dose DHA were more likely to deliver at 40 weeks or beyond, suggesting the possibility of prolonged gestation in that group.

When the researchers modeled what might happen if DHA dosing were matched to genotype — higher doses for I-allele carriers, standard doses for D/D carriers — the combined group showed the highest proportion of births falling into the healthy 37-to-40-week range.

Why Ancestry Keeps Coming Up

Black participants in the study had both higher rates of preterm birth overall and a higher prevalence of the I allele, and they appeared to benefit most from the higher DHA dose. The authors frame this as a potential piece of a larger puzzle: some of the documented racial disparity in preterm birth outcomes might be connected to inherited differences in fatty acid metabolism, though this study doesn't establish that as a settled explanation.

Reading the Results With Appropriate Caution

The authors are upfront that this work has real limits. The genotype comparisons were not something the original trial was designed to test, and in some subgroups the number of actual preterm birth events was small, which makes the estimates less stable. The researchers describe their findings as hypothesis-generating rather than conclusive, and they call for future trials specifically designed to test genotype-guided DHA dosing before it could be considered a reliable strategy.

Where This Leaves Expecting Parents

For now, this research points toward a genetic test that doesn't yet exist in routine prenatal care, tied to a dosing strategy that hasn't been formally validated. It's a meaningful step toward personalized nutrition during pregnancy, but it isn't a reason to adjust DHA intake on your own. Anyone considering omega-3 supplementation during pregnancy should talk with their obstetrician or midwife about what dose, if any, makes sense for their individual situation.

Source: PLOS Medicine, DOI: 10.1371/journal.pmed.1005274, based on a secondary analysis of the ADORE randomized trial by Li, Kothapalli, Carlson, Gajewski, DeFranco, and Brenna.