Fish oil has been marketed to women as a prenatal supplement.
But a 10-year study of more than 2,500 pregnant women found it did not improve babies' intelligence, according to the Australian research.
The findings suggested that fish oil might lead to slightly longer pregnancies, but that would require further research, the team said.
"If a pregnant woman has a healthy, balanced diet, then the baby's brain development is not going to benefit from having these fish oil supplements," study co-author Dr Jacqueline Gould told the BBC.
The research, by the South Australian Health and Medical Research Institute, followed women from their pregnancy to when their children were seven years old.
Participants were given either a daily fish oil supplement or a placebo.
The results found taking a 800mg dose of docosahexaenoic acid (DHA), an omega-3 acid, had no impact on babies' intelligence.
Dr Gould said supplement makers had specifically marketed fish oil as a way to boost development in infants.
She said the research, detailed in the Journal of the American Medical Association, showed fish oil might be linked with longer pregnancies.
"What that meant was, there were slightly fewer children born pre-term," Dr Gould said.
"We're currently trying to validate that in a much bigger study."
Taking fish oil while pregnant does not improve the brain development of babies, new research has found.