Children whose mothers eat fish regularly during pregnancy develop better language and communication skills, research suggests.
Fish can help brain development
The findings are based on analysis of the eating habits of 7,400 mothers.
A fish diet seemed to lead to better language and social skills in children as young as 15 months.
The research, by the University of North Carolina, published in the journal Epidemiology, also looked at mercury levels in umbilical cords.
In the US, mothers-to-be are warned against eating more than 12 ounces of fish
a week because of the effect of mercury poisoning on their unborn child.
But the researchers found that in the UK, where mercury levels in seafood are relatively low, the benefits of eating fish in moderation outweighed the risk of contamination.
Julie Daniels, from the University of North Carolina, studied women taking part in the Children of the 90s project based at the University of Bristol.
She found that 70% of the women said they ate fish at least once a week.
The largest effect of a fish diet seemed to be in children's understanding of words.
In tests carried out at 15 months children whose mothers ate fish at least once a week scored 7% higher than those whose mothers never ate fish.
Similar but less marked patterns were seen in tests on social activity and language development.
Scores were also higher among children who ate fish at least once a week before their first birthdays.
Ms Daniels said: "Fish is a source of many nutrients that can be beneficial during pregnancy, as well as a source of contaminants such as methyl mercury.
"Previous investigations of fish intake in relation to neurodevelopment have focused on possible damage from contaminants while the potential benefits have been relatively unexplored."
The researchers examined the umbilical cords of 1,200 babies for the presence
of mercury and found overall levels were low.
While they found higher concentrations in women who ate fish, there was no
link found in the developmental tests.
The researchers also found that eating four ounce portions of fish between one and three times a week was enough to produce benefits. Higher levels of consumption did not seem to have any greater effect.
The researchers conceded that more research was needed to assess the benefits of eating fish in areas with a higher level of mercury pollution.
Dr Anne Nugent, a nutrition scientist at the British Nutrition Foundation, told BBC News Online certain types of oily fish such as salmon and tuna contained omega-3 fatty acids, which were necessary for the normal growth and development of the foetal brain and nervous system.
"Whilst this study does not specify how often the mothers ate oily fish, whether the infants were breast or bottle-fed or whether after weaning the infants consumed fish as part of their diet, it does suggest that eating fish during pregnancy has beneficial effects.
"More studies are needed to confirm the effects observed on cognitive function."
The UK Food Standards Agency recommended on Thursday that women of childbearing age should eat a maximum of two portions of oily fish a week.
Others can eat up to four portions a week.