Nonetheless, several experts said they would continue to support taking DHA in pregnancy, especially since it is safe and apparently has few downsides. They cited a smaller 2003 Norwegian study that found I.Q. increases at age 4, although no cognitive benefit was seen in infants or 7-year-olds.

That study’s lead author, Dr. Ingrid Helland of Oslo University Hospital, said that perhaps the benefit shown at age 4 existed at 7 but was difficult to identify amid other developmental factors. She said babies in her study might have received more DHA than those in this study. Dr. Oken said the new study might have shown no benefit in 18-month-olds because effects in infants might be “hard to measure” or “it may be that the benefit of DHA is not as important as when you’re 4.”

Dr. Makrides’s team plans assessments at 4 and 7. Dr. Jatinder Bhatia, a neonatologist heading the nutrition committee of the American Academy of Pediatrics, said that unless further studies showed no benefit as children got older, he would continue recommending DHA, now in many prenatal vitamins, because “I’m not convinced at this time that we should reverse course.”

Much about DHA is unknown. Its effect in supplements for children and adults is being studied, as well as whether supplements offer the same benefit as DHA-rich fish, like salmon.

Some studies, including the new report, suggest DHA supplementation in pregnancy reduces the likelihood of premature birth. And the new study showed small reductions in postpartum depression in women with histories or high risk of depression. Dr. Scott Stuart, a University of Iowa psychiatry and psychology professor, said his pregnant patients with mild to moderate depression had improved when taking DHA while seeing a counselor.

“There’s no harm that we know of at all, in contrast to many antidepressants,” he said. “And it might be of some help.”