Australian researchers found women who took daily fish oil supplements in the second half of pregnancy had the same risk of postnatal depression as those who did not.
Likewise, fish oil had no effect on the cognitive or language ability of children born to mothers who took the capsules.
The results suggest fish oil supplementation during pregnancy may not be necessary.
Previous studies suggested higher levels of omega-3 fatty acids in seafood reduced levels of postnatal depression and improved brain development.
Docosahexaenoic acid (DHA) was thought to be key to this effect, but interventional trials have produced mixed results.
To test the effect of DHA on outcomes, researchers recruited 2,399 women who were less than 21 weeks pregnant. Women were randomised to receive 800mg per day of DHA or control capsules. They were scored for depression symptoms during the six months postpartum. A total of 726 children were tested for cognitive and language development at 18 months of age.
Researchers found mean scores for both depression and child development were the same between the two groups.
Researchers concluded: 'The results of the trial do not support routine DHA supplementation for pregnant women to reduce depressive symptoms or to improve cognitive or language outcomes in early childhood.'