Washington ? Claims touting a component of fish oil as a mood enhancer and a spur to infant brain development may be a bit fishy, a new study suggests.

DHA, an increasingly common ingredient in prenatal vitamins and baby formula and taken as a supplement by pregnant women, failed to prevent postpartum depression or to enhance babies’ cognitive development or language acquisition, a large study has shown.

The finding, reported Tuesday in the Journal of the American Medical Association, casts new doubt on a dietary supplement whose promise as brain-food has been aggressively marketed despite inconsistent results.

A welter of studies over the last decade has found that pregnant or breastfeeding women with higher blood concentrations of DHA or docosahexaenoic acid, a fatty acid, are less likely to suffer depression and more likely to have babies with better attention spans, sharper vision and earlier mastery of many skills. Most of the studies were small, preliminary and contradicted by later research — but they touched off a frenzy of commercial hype by makers of baby food and formula and prompted public health agencies throughout the world to recommend that pregnant and lactating women take in more DHA.

Those guidelines, the JAMA article authors wrote, came despite “uncertainty about the benefits” for pregnant women and babies.

In the new study, 2,399 women at the midpoint of their pregnancies were divided into two groups. One took a daily capsule of 800 mg of DHA derived from fish oil until giving birth; the other took an identical capsule filled with vegetable oil.

Neither the women who got the DHA supplement nor the children they bore fared better than the moms and babies who took vegetable oil capsules.

The researchers found DHA supplementation to be safe for mother and child. But, they added, “The results do not support routine DHA supplementation for pregnant women to reduce depressive symptoms or to improve cognitive outcomes in early childhood.”