Concerns about the negative effects of mercury on fetal development have led to
the US National Research Council
warning against eating too much fish during pregnancy, but those guidelines may need to be reviewed.

New research instead found that fish accounts for only seven percent of mercury levels in the human body. Further, an analysis of 103 food and drink items consumed by 4,484 women during pregnancy found that the 103 items together accounted for less than 17 percent of total mercury levels in the body. 

Previous research from the Children of the 90s study at the University of Bristol
showed that eating fish during pregnancy has a positive effect on the IQ and eyesight of the developing child, when tested later in life. Exactly what causes this is not proven, but fish contains many beneficial components including iodine and omega-3 fatty acids. 

Stepwise linear regression results for a model of dietary variables that were positive predictors of untransformed total blood mercury (model 1) and a model that included both positive and negative predictors of total blood mercury (model 2). Credit: 
doi.org/10.1289/ehp.1206115

After fish (white fish and oily fish), the foodstuffs associated with the highest mercury blood levels were herbal teas and alcohol, with wine having higher levels than beer. The herbal teas were an unexpected finding and possibly due to the fact that herbal teas can be contaminated with toxins. 

Another surprise finding was that the women with the highest mercury levels tended to be older, have attended university, to be in professional or managerial jobs, to own their own home, and to be expecting their first child. Overall, however, fewer than one percent of women had mercury levels higher than the maximum level recommended by the US National Research Council. There is no stated safe level issued by the UK government. 

The authors conclude that advice to pregnant women to limit seafood intake is unlikely to reduce mercury levels substantially. Speaking about the findings, the report's main author, Professor Jean Golding OBE, said, 'We were pleasantly surprised to find that fish contributes such a small amount (only seven per cent) to blood mercury levels. We have previously found that eating fish during pregnancy has many health benefits for both mother and child. We hope many more women will now consider eating more fish during pregnancy. It is important to stress, however, that pregnant women need a mixed balanced diet. They should include fish with other dietary components that are beneficial including fruit and vegetables.'

Citation: Jean Golding, Colin D. Steer, Joseph R. Hibbeln, Pauline M. Emmett, Tony Lowery and Robert Jones, ‘Dietary predictors of maternal prenatal blood mercury levels in the ALSPAC birth cohort study', Environmental Health Perspectives, doi.org/10.1289/ehp.1206115