Women should consider eating more fish during pregnancy as concerns over mercury levels may be unfounded, UK scientists say.
Mothers-to-be are currently warned to limit their fish intake due to the negative effects of mercury on foetal development.
But scientists say these official guidelines may need to be reviewed following a study by the University of Bristol.
The research suggests fish accounts for just 7 per cent of mercury levels in the human body - with all food and drink totalling less than 17 per cent.
Surprisingly, it also found herbal teas and alcohol to be the foodstuffs associated with the highest mercury blood levels, after white and oily fish.
Food including French fries, white bread and milk were found to have a "protective effect".
Professor Jean Golding OBE, lead author of the report published in Environmental Health Perspectives, said eating fish had health benefits for mother and baby.
"We were pleasantly surprised to find that fish contributes such a small amount, only 7 per cent, to blood mercury levels," Professor Golding said.
"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."
The study involved 4484 women taking part in the Children of the 90s study at the University of Bristol, who filled in questionnaires about their food intake during pregnancy.
Blood samples were also taken and analysed for traces of mercury, which can adversely affect a baby's growing brain and nervous system in the womb.
Results showed that 38 of the women - less than 1 per cent - had mercury levels higher than the maximum recommended by the US National Research Council. There is no official safe level in the UK.
Women with the highest mercury tended to be older, have attended university, to be in a professional job, own their home and be expecting their first child.
Researchers then analysed 103 food and drink items consumed by the women during pregnancy and found the items accounted for less than 17 per cent of total mercury levels in the body.
The levels were most associated with oily fish, white fish, herbal tea, alcohol, boiled rice, fresh fruit, sunflower or other oils used for frying and pasta.
Other foods included pure fruit juice, "health" foods, brown or granary bread, pulses, shellfish, bran cereals and salad.
Foods that were significant negative predictors of blood mercury levels included white bread and French fries.
A spokeswoman for the University of Bristol added: "The herbal teas were an unexpected finding and possibly due to the fact that herbal teas can be contaminated with toxins."
Mercury can also be absorbed from water and air and is present in dental amalgam fillings, beauty products, legal drugs such as alcohol and tobacco, illegal drugs and medications.
The major sources of mercury include refuse incineration, fossil fuel combustion and fungicides and pesticides.
Previous research by Children of the 90s showed eating fish during pregnancy has a positive effect on the IQ and eyesight of the developing child, when tested in later life.
It is not known exactly what causes the benefits, though fish contains components including iodine and omega-3 fatty acids.
"The authors conclude that advice to pregnant women to limit seafood intake is unlikely to reduce mercury levels substantially," the spokeswoman added.
The report, Dietary predictors of maternal prenatal blood mercury levels in the ALSPAC birth cohort study, is published in the journal Environmental Health Perspectives.
