Spanish researchers measured omega-3 in the cord blood of 953 newborns, then tested the children until they were 15. The fish omega-3 and the plant omega-3 went with different things at different ages, and one of them faded

The blood left in the umbilical cord after a birth is a record of what reached the baby in the last stretch of pregnancy. Between 2003 and 2008 a Spanish project called INMA, short for Infancia y Medio Ambiente (childhood and environment), recruited 2,644 pregnant women in four regions and kept cord blood from many of the births. Years later, researchers measured three omega-3 fatty acids in 953 of those samples.

The three were DHA and EPA, which come mostly from fish and seafood, and ALA, the plant omega-3 found in walnuts, flaxseed and some vegetable oils. Then they went back to what the children had done since. The same children had sat computer tests of attention at about 5, 7 and 11, tests of working memory at 7, 11 and 15, and reasoning puzzles at about 9 and 15. The results came out in the European Journal of Epidemiology on 8 October 2026.

Averaged over the whole of childhood, two findings stood out. Children whose cord blood was in the top third for DHA were less inattentive than those in the bottom third, by 0.20 of a standard deviation, and had better working memory, by 0.24. Children in the top third for ALA were also less inattentive, by 0.19. EPA was linked to nothing at all, and none of the three was linked to reasoning scores overall.

A fifth of a standard deviation is small. It is a difference you can see across several hundred children and would not notice in one child.

The more interesting part is what happened with age. For ALA, the advantage sat at the start. The model put high-ALA children well ahead on attention at 5 and 7, and then the gap closed and reversed: by 15 the high-ALA group was the more inattentive one. Reasoning followed the same shape, ahead early and behind at 15.

DHA ran the other way. On attention, high-DHA children were no better at 5, a little better at 7, and the gap kept widening to 15. On reasoning they started behind and finished ahead. Working memory was the exception: the DHA advantage was largest in the early years and had mostly gone by 15.

The authors read this as two fats doing different jobs, ALA giving early support and DHA helping the slower maturing of attention and reasoning. That is one reading. Another is more cautious. The study tested three fatty acids against three kinds of thinking at several ages, and a pattern where an early advantage flips into a later disadvantage is the kind of result that needs to turn up again in another cohort before anyone leans on it.

There are plainer limits too. Nobody gave these mothers omega-3; the study only observed what was in the cord. Mothers with more DHA in their blood eat more fish, and families who eat more fish differ in other ways. The researchers adjusted for the mother's age, education, weight, alcohol and overall diet, and in extra checks for her verbal IQ, breastfeeding, the child's own fish and nut intake and mercury, and the results held. They still say plainly that leftover confounding 'remains a fundamental issue'. Cord blood also reflects the weeks before birth, not the whole pregnancy, and no blood was taken from the teenagers, so their own diet since can't be separated out.

One line in the funding section is worth knowing about. Alongside Spanish and European public funds, the California Walnut Commission gave support, and walnuts are a leading source of ALA. The authors declare no conflicts of interest, and the ALA result that lasted to 15 was not a flattering one.

None of this changes the standard advice, which is that pregnant women benefit from eating fish that is low in mercury a couple of times a week. What it adds is a reason to be patient with the science. A study that stopped at age 7 would have told a different story from one that ran to 15.