Women who eat fish during pregnancy have brighter children, according to a study.
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The evidence suggests that advice to expectant American mothers to limit fish consumption for fear of mercury poisoning is misguided.
The study supports the contrary advice, given by the Food Standards Agency in the UK, which backs fish as a healthy food. The FSA simply advises mothers to avoid shark, swordfish and marlin, and restrict their intake of tuna.
The new research into children's behaviour and intelligence suggests that women who follow the US "advisory" issued in 2004 to limit consumption, or cut fish out of their diet altogether, may miss nutrients that the developing brain needs - and so harm their children.
The findings come from a study of almost 9,000 British families taking part in the Children Of The 90s project at the University of Bristol.
The lead researcher, Joseph Hibbeln of the US National Institutes of Health, and the Bristol scientists, including Professor Jean Golding, compared the amount of fish eaten by pregnant mothers with the development of their children up to the age of eight.
Seafood - fish and shellfish - is the predominant dietary source of long-chain omega3 fatty acids, which are essential for development of the nervous system.
Middle-class women are more likely to eat fish, but even after adjusting for social class and 27 other factors, including breast-feeding, the link between fish and children's development held true. This suggests that fish-eating is not simply a marker for social class.
Mothers who ate more seafood than the US guidelines (340 grams, or three portions a week) had children who were more advanced in development tests measuring fine motor, communication and social skills as toddlers, had more positive social behaviours and were less likely to have low verbal IQ scores at the age of eight. Those children whose mothers had eaten no fish were 28 per cent more likely to have poor communication skills at 18 months, 35 per cent more likely to have poor fine motor coordination at age three and a half, 44 per cent more likely to have poor social behaviour at age seven and 48 per cent more likely to have a relatively low verbal IQ at age eight, when compared with children of women who ate more than the US guidelines advised.
Dr Hibbeln said: "We have found that when women had low levels of seafood consumption, the outcome is exactly the opposite of what was assumed by the United States advisory. Unfortunately, the advice appears to have had the unintended consequence of causing harm in a specific developmental domain - verbal development - where protection was intended. We recorded no evidence to lend support to the warnings of the US advisory that pregnant women should limit their seafood consumption.
"In contrast, we noted that children of mothers who ate small amounts, 340g per week, of seafood were more likely to have sub-optimum neuro-developmental outcomes than children of mothers who ate more seafood than the recommended amounts."
The findings are published in this week's issue of The Lancet.
Sue Macdonald, the education and research manager at The Royal College of Midwives, said: "Midwives are very aware of the importance of advising women and their families of the benefits of eating fish - including fishes high in omega - beneficial both for brain development of the growing baby, and for long-term health benefits for a healthy heart.
"But it is also important that women are aware that they should avoid raw seafood - especially raw shellfish.
"At present they should also avoid shark, marlin and swordfish as there has been a suggestion that this might carry higher levels of mercury.
"I would advise women to talk to their midwife - who can give them realistic advice."
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The full abstract of the study can be found at the PubMed website.