Eating oily fish and seeds in pregnancy can boost children's future brain power and social skills, research suggests.
60% of the brain is formed of fats
A study of 9,000 mothers and children in Avon suggested those who consumed less of the essential fatty acid Omega-3 had children with lower IQs.
These children also had poorer motor skills and hand-to-eye co-ordination, research in the Economist said.
The Food Standards Agency says pregnant women should consume only one or two portions of oily fish a week.
A team from the National Institutes of Health in the US analysed data from a long-term study done in Avon, UK.
Looking at the effects of Omega-3 intake on 9,000 mothers and their children, the team found mothers with the lowest intake of the essential fatty acid had children with a verbal IQ six points lower than the average.
While those with the highest consumption of mackerel and sardines and other sources of Omega-3 had children, at age three-and-a-half, with the best measures of fine-motor performance, researchers said.
Low intake of the crucial fatty acid also appeared to lead to more problems of social interactions - such as an inability to make friends.
Research leader Dr Joseph Hibbeln said "frightening data" showed 14% of 17-year-olds whose mother had eaten small quantities of Omega -3 during pregnancy demonstrated this sort of behaviour.
This compared with 8% of those born to the group with the highest intake, he said.
Dr Hibbeln said: "The findings of poor social development and poor motor control in children indicate that these children may be on a developmental trajectory towards lifelong disruptive and poorly-socialised behaviour as they grow up."
Professor Jean Golding of Bristol University set up the original research - the Avon Longitudinal Study of Parents and Children -15 years ago to look at the predisposition to disease.
She told the BBC: "The baby's brain needs Omega-3 fatty acids. It doesn't create its own fatty acids so it needs to be something that the mother will eat."
The new research also builds on earlier work in the US which suggests pregnant mothers will develop children with better language and communication skills if they regularly consume oily fish.
Nutritional expert Patrick Holford, director of the Brain Bio Centre, said Omega-3 was key to children's intelligence because the brain is formed of 60% fat - 30% of which is essential fats.
Successive studies have shown clear links between intelligence and consumption of this essential fatty acid, he added.
"It's absolutely essential that pregnant women take in enough Omega-3 and that children in early infancy take in enough Omega-3."
The richest sources of Omega-3 are larger fish which eat other fish, but research shows that the larger the fish the more pollutants, such as mercury, they contain.
For this reason Mr Holford recommends women consume two portions of wild or organic salmon, trout or sardines weekly.
Seeds such as flax, pumpkin and hemp are good sources of Omega-3 for vegetarians, but large quantities need to be consumed to gain the same effect.
This might translate to two tablespoons of seeds daily, Mr Holford said, but women can also use a high quality Omega-3 supplements.