A review of the existing evidence finds it to be inconclusive about whether omega-3 fatty acids taken by mothers during pregnancy boost their kids' brain development early in life.
"There are so many trials where pregnant women are supplemented with omega-3 fatty acids and they've all got different results," said lead study author Jacqueline Gould, a researcher at the Women's and Children's Health Research Institute in Adelaide, Australia. "We found that there was neither a positive nor a negative effect on visual or neurological outcomes."
The Australian team, who published their findings in The American Journal of Clinical Nutrition, analysed data from 11 clinical trials with a total of 5272 participants who were randomly assigned to take omega-3 supplements or placebos during the last half of their pregnancies.
Across the trials, the amount of omega-3 taken by the mothers ranged from 240 to 3300 milligrams per day. And the ages at which children's brain and vision development were assessed ranged from newborn to 7 years old.
According to the researchers, most of the clinical trials included too few participants to distinguish subtle differences expected from nutritional studies, excluded complicated pregnancies (in which greater differences might have been seen) and didn't follow the children long enough during development.
"Our analysis highlights that more research is needed," Gould told Reuters Health.
Omega-3 fatty acids are crucial for healthy foetal brain development and are commonly found in fatty fish such as tuna, mackerel and sardines. Human brains and eyes contain large amounts of docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), both forms of omega-3.
Developing foetuses can get DHA from their mothers' fat stores, and from food and supplements they consume during pregnancy.
The hope that omega-3 supplements might enhance brain development stems from research like a large study from Denmark that found mothers who reported eating more fish had children with greater neurological and motor development in their first months.
The author of that study, Dr. Sjurdur Olsen, head of the Center for Fetal Programming in Copenhagen, cautions, however, that in Denmark at least, mothers who eat more fish tend to be better educated and more well off - which could both be important factors in a child's development.
Other studies have found that an expecting mother's fish oil intake doesn't increase her child's IQ or enhance her baby's visual development.
Still, several mainstream organisations, including the European Food Safety Authority and the World Health Organisation, do endorse omega-3 supplements for expectant mothers, noted Harry Rice, vice president of regulatory and scientific affairs at the Global Organisation for EPA and DHA Omega-3s, a trade organisation.
"It's because of the science that many organisations and government agencies around the world recommend DHA supplementation for pregnant and lactating women," Rice told Reuters Health in an email.
It may be too soon to reach a definitive conclusion on this topic, according to Elvira Larqué, physiology professor at the University of Murcia in Spain, who independently reviewed DHA clinical trials in a 2012 study.
Larqué agreed that more trials are needed. The ones analysed in the new report had weaknesses, including the fact that all sources of DHA were not accounted for in the mothers' diets, and measures of intelligence were based on tests, such as observing a child play with a toy, that were too subjective.
Olsen said the discrepancy between official encouragement for mothers to take prenatal DHA supplements, and the actual evidence for their effects probably stems from wishful thinking.
"People want to have some good news," he said. "There's a strong wish to have simple means to get strong effects."