In a new study on pregnant women, eating fish like tuna is linked to protective effects on cognitive development and autism symptoms.

In a surprising new study, some of the very fish pregnant women are discouraged by some health groups from eating may be the ones associated with the most protective effects on fetal brain development.

The observational study, funded by the government of Spain and published in the American Journal of Epidemiology, looked at approximately 2,000 pregnant mothers across Spain. During pregnancy, the women reported their fish intake via food questionnaires that categorized intake by types of fish. Most of the women ate some fish during pregnancy; the average amount was three servings a week. During birth, blood from the women’s umbilical cords was assessed for levels of mercury, a contaminant linked to neurotoxic effects, and DHA, an omega-3 fatty acid. After birth, the women’s children were tested on scales for cognitive development and a scale measuring symptoms indicative of autistic spectrum disorder, both when they were 14 months old and five years old.

Eating more servings of seafood a week was associated with increases in cognitive scores and decreases in symptoms of autistic spectrum in the children. Eating 600 grams of total fish per week—about three to four servings—was linked to a 2.8 point increase in IQ score. Unexpectedly, the protective effect was particularly strong for large fatty fish like tuna, which have some of the highest levels of DHA—and mercury—among fish types.

The umbilical cord blood test revealed higher amounts of mercury and DHA for people who ate more large fatty fish, but researchers didn’t see negative associations with mercury and the child’s neurodevelopment. “It seems that our mercury indicator is telling more about fish consumption, and the positive effect of fish consumption, than the neurotoxic effects of mercury,” says study co-author Jordi Julvez, research fellow at the Centre for Research in Environmental Epidemiology (CREAL) in Barcelona. The benefits tapered off when fish consumption was higher than 600 grams.

How much fish is safe for pregnant women—and what kind—is hotly debated. While the European Food Safety Authority recognizes a benefit to one to four servings of fish per week for mothers-to-be, it recommends limiting fish high in mercury. In the United States, last year’s draft recommendations from the U.S. Food and Drug Administration (FDA) revised their stance on eating fish during pregnancy. Previously, they’d cautioned women not to eat too much of it. In the new guidelines, they encouraged women to eat more of it—but only 2-3 servings a week—and to choose the types lower in mercury and limit their consumption of big predatory fish, such as tuna, which have more mercury.

The study was observational, so it wasn’t designed to determine a cause. But Julvez speculates that DHA omega-3s during pregnancy are the key—especially since other biomarkers for fish intake that they looked at, like vitamin B, didn’t explain the association. DHA is important in building neurons and cell membranes, he says. And pregnancy seems the most effective time for children to reap the benefits on brain development. “In that specific moment, a large amount of DHA is needed when the brain is growing,” Julvez says.

More research is needed, especially on the role of mercury and whether the positive brain effects last past age 5. But for now, the results suggest that current American recommendations may be too stringent, Julvez and his co-authors write. “Overall, the present results suggest no adverse associations of high seafood consumption in pregnancy with offspring neurodevelopment,” they write—and that high seafood consumption may even bring some brain benefits.