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Benefits of oily fish in pregnancy outweigh risks

Obstetrics and gynaecology

Obstetrics and gynaecology

A new large and well conducted study has suggested that a diet low in fish during pregnancy poses greater risks to the development of the fetus than the risk of possible exposure to trace amounts of mercury.

There have been concerns for some time that the oily fish in our diet has become contaminated with trace levels of neurotoxins such as methyl-mercury. This heavy metal can interfere with neurological development and the worry has been that pregnant women who consume fish (in particular oily or dark meat fish) might be risking the future health and wellbeing of their offspring.

The UK Food Standards Agency has issued advice on limiting the number of portions of oily fish consumed each week by pregnant women. However, oily fish are an excellent source of essential long-chain omega-3 fatty acids, and these nutrients are required for satisfactory neurological development in the fetus.

The study recruited 11,875 mothers in the Bristol area who gave birth between March 1991 and January 1993. It investigated fish consumption in pregnancy and the child's subsequent social, emotional, motor and intellectual development over several years.

Fish intake data were collected using validated food frequency questionnaires. The quantity and type of fish (white, oily or shellfish) were recorded. Child development data were collected using a combination of questionnaires at six, 18, 30, 42 and 81 months, and a child IQ test at eight years.

The data were very carefully processed using regression analyses to control for potential confounding factors, such as maternal smoking and alcohol and socio-economic background.

The findings were clear. Mothers who consumed the most seafood (>340g/week), irrespective of the proportion of oily fish, had children with the best outcomes compared with women who ate less than 340g/week. Women who consumed no fish at all during pregnancy had the highest risk of having a child with the poorest neurodevelopment, behaviour and intelligence scores.

This study illustrates why there is a hierarchy of clinical study types; the strength of evidence from large observational studies are considered more reliable than expert opinion.

Hibbeln JR, Davis JM, Steer C, et al. Maternal seafood consumption in pregnancy and neurodevelopmental outcomes in childhood (ALSPAC study): an observational cohort study. Lancet 2007;369:578-85

Reviewer

Dr Chris Barclay
GP, Sheffield

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