A bigger omega-3 dose in pregnancy cut very early births for women with one gene variant, and may have done the opposite for women without it. The whole case rests on 17 babies

In the US, the usual dose of DHA in pregnancy is about 200 mg a day, the amount recommended to support the baby's developing brain. A US trial called ADORE asked whether five times that much would also keep babies from arriving too soon. Its main results, published in 2021, said probably yes for births before 34 weeks, mostly in women who started pregnancy low in DHA. A new paper in PLOS Medicine, published on 8 October 2026, goes back to the same women with a different question: does the answer depend on their genes?

The gene in question is a short stretch of DNA in the FADS cluster that some people have (the 'insertion', or I) and some don't (the 'deletion', or D). People with the insertion make more arachidonic acid, an omega-6 fat the body turns into prostaglandin E2, one of the compounds that sets labour going. DHA gets in the way of that step. So the team's reasoning ran like this: women who make more arachidonic acid may be more prone to early labour, and may need more DHA to hold it back.

Of the 1,032 women who finished ADORE, 761 had agreed to genetic testing and had complete records. Their white blood cells had sat in a freezer at −80 °C until late 2023, when they were shipped to the University of Texas at Austin and tested for this one variant by people who knew nothing about how the pregnancies had gone. A little over half the women, 419 of 761, carried at least one copy of the insertion.

Among carriers, the dose looked as if it mattered. On 200 mg a day, 9 of 214 had their baby before 34 weeks. On 1,000 mg, 2 of 205 did. The authors put the probability that the higher dose really lowered that risk at 0.99. Births before 37 weeks fell too, from 31 of 214 to 19 of 205.

Among the women with no copy of the insertion, things went the other way on two counts. Births at 40 weeks or later were more common on the high dose: 49 of 191, against 23 of 151 on the low dose. And very early births were, if anything, more frequent: 5 of 191 on the high dose and 1 of 151 on the low dose. The authors say this last result should be read cautiously because of the small numbers, and they point to an Australian trial, ORIP, that saw a similar hint in women who already had plenty of omega-3.

Now count the births this rests on. Across all 761 women there were 17 before 34 weeks. Split by gene and dose, the four groups had 9, 2, 1 and 5. The headline contrast, the 'interaction' between gene and dose, is a comparison of those four numbers. A statistical model can return a probability of 0.99 from them, but two or three babies born a week later would have changed the picture. The authors tested this themselves: the link between the variant and births before 37 weeks stopped clearing their own bar if as few as three or four of the 31 cases were shifted.

It also matters that nobody planned this analysis when the trial was designed. The paper says so plainly, several times. In its favour, only one variant was ever tested, it was chosen in advance for a biological reason, and within each gene group the dose was still assigned at random. That is a good deal better than trawling thousands of genes for whatever turns up. It is still one look back at one trial.

There is a wider thread here too. The insertion is much more common in some ancestries than others. In this study 133 of the 156 Black participants carried it, compared with 286 of the 605 other participants, and the authors note it is also common in people of South Asian and African descent. Black women in the US have higher rates of preterm birth, and the paper suggests this variant, combined with low DHA intake, could be one piece of that. It is a hypothesis worth testing, and it doesn't replace the social and medical causes others have documented.

This gene test isn't part of routine pregnancy care, and the paper isn't a reason to ask for one or to change a supplement. What it does is give the next trial a clear job: test the variant first, assign the dose by genotype, and see whether the babies arrive closer to term. Until then the authors' own practical note is a modest one. Where the genotype is unknown, they say it is prudent to stop DHA supplements at 37 weeks to lower the chance of running past the due date, in line with a 2024 clinical guideline they cite.