Behind the headlines: Does eating fish cause birth risk?

Mercury in fish might increase a woman's chances of giving birth prematurely. By Emma Baines

What is the story?

Eating too much fish during pregnancy could increase the risk of giving birth prematurely, according to media reports.

The papers reported that women who ate a lot of fish were exposed to higher levels of mercury, which can trigger premature birth.

In addition, eating fish contaminated with mercury during pregnancy could stop the baby’s nervous system from developing properly, leading to difficulties with learning, memory and attention in later life, the papers added. They warned that women should avoid eating oily fish such as mackerel, salmon and sardines during pregnancy because these were most likely to con- tain high levels of mercury.

Women should take fish oil supplements instead of eating fish while they are pregnant to reduce the risk of mercury contamination, the papers suggested.

What is the research?

The media reports are based on a US study which looked at the relationship between fish consumption, mercury exposure and premature birth.

The researchers wanted to see if the benefits of a high omega-3 fatty acid intake from eating more fish would outweigh the risks associated with increased exposure to methylmercury.

The study included 1,024 women from 52 prenatal clinics in Michigan who were recruited to the study between the 15th and 27th week of pregnancy.

They gave information on the amount and type of fish they consumed during the pregnancy and gave a hair sample to be tested for exposure to mercury.

The hair samples revealed how much mercury the women had been exposed to during the previous six months.

The researchers found that women who ate more fish had higher levels of mercury in their hair.

Those who ate fish more than 24 times over the first six months of pregnancy had an average of 0.25µg/g mercury in their hair compared to 0.11µg/g in those who did not eat fish at all.

Shell and canned fish, bought fish and fish caught from local rivers were all associated with increased mercury exposure. The researchers then looked at the effect of mercury exposure on the risk of preterm delivery.

They found that the 44 women who gave birth before 35 weeks were three times as likely as women who gave birth at term to fall into the highest category of mercury exposure, which was 0.55µg/g or higher.

What do the researchers say?

Lead researcher Dr Claudia Holzman, associate professor at the Department of Epidemiology at Michigan State University said: ‘We showed a relationship between high levels of total mercury in maternal hair and risk of delivery at less than 35 weeks’ gestation. The relationship between fish consumption and risk of preterm delivery is a separate question because fish consumption may have both beneficial effects and pose a hazard due to pollutants.

‘The relationship of benefit to risk may depend on type of fish, the level of contaminants in the fish, the amount of fish consumed, and other factors,’ she said.

Dr Fei Xue, from Harvard school of Public Health, who was also a researcher on the study, added that this was the first time a relationship between mercury level and preterm delivery had been found.

‘We really need to wait for other studies to confirm these results before we can draw any conclusions, but fish consumption has been well established as a source of methylmercury so, during pregnancy, women should avoid consumption of fish high in the food chain such as shark and sword fish, which have the highest mercury levels,’ said Dr Xue.

‘Women should also reduce their consumption of canned fish to improve pregnancy outcomes.’

What do other experts say?

Dr Toni Steer, nutritionist at the Medical Research Council Human Nutrition Research Centre in Cambridge said the results of the study were not conclusive enough to justify advising pregnant women to cut out all oily fish from their diet.

‘The group of women in this study who had preterm deliveries was small, so it is hard to draw any firm conclusions,’ she said.

She added that nutritionists have been aware for some time there are risks associated with high mercury levels, but that the level of mercury exposure from the recommended weekly intake of oily fish would be so low that it would not present any danger.

‘The FSA recommends that when pregnant you should eat no more than two portions of fresh oily fish a week. The dietary benefits of oily fish far outweigh the risks.’

Informing patients

A US study found that women who gave birth prematurely were more likely than other women to have high levels of mercury from eating fish.

Research has suggested that eating fish during pregnancy could improve birth outcomes.

Mercury levels from eating two portions of oily fish a week would be too low to cause any harm.

Pregnant women should continue to eat oily fish.

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