The report comes the same week as the U.S. Environmental Protection Agency doubled its estimate of how many newborns had unsafe levels of mercury in their blood.
The study, done by an international group led by researchers at the Harvard School of Public Health, also showed that children exposed to mercury in the womb may suffer permanent damage to their heart function.
\"We found that both prenatal and postnatal mercury exposure affects brain functions and that they seem to affect different targets in the brain,\" Philippe Grandjean, who led the study, said in a statement.
Grandjean and colleagues studied more than 1,000 mothers and children living in Denmark\'s Faroe Islands. Residents there eat large amounts of fish, much of it contaminated with mercury.
They measured mercury in umbilical cord blood taken from the children at birth and then in hair samples taken at ages 7 and 14.
Most of the mothers were suffering from mercury contamination, with their own hair levels at childbirth on average above 1 microgram per gram, the limit recommended by the EPA and the independent, nongovernment National Research Council.
BRAIN SIGNAL IRREGULARITIES
Writing in the Journal of Pediatrics, Grandjean and colleagues in Denmark and Japan said they put electrodes on the heads of the children to measure electrical signals in the brain. They found delays in brain signaling, and the higher the mother and child\'s mercury load at birth, the more distinct the irregularities.
They also found these neurological changes affected heart function. The children with the most mercury in their blood were less capable of maintaining the normal variability of the heart rate needed to secure proper oxygen supply to the body, Grandjean\'s team found.
Earlier this week an EPA researcher published a report doubling the estimates of how may U.S. infants have unsafe levels of mercury in their blood.
The researcher, Kathryn Mahaffey, estimated that 630,000 infants were born in a 12-month period between 1999 and 2000 with blood mercury levels higher than 5.8 parts per billion, the EPA\'s level of concern. This is more than double the previous estimate of 300,000 infants.
\"It is important to note that this estimate is preliminary in nature, and is based on recently available information about mercury in umbilical cord blood versus maternal blood,\" Mahaffey said in a statement.
\"EPA is still reviewing these new studies and their potential implications.\"
Her full study is available on the Internet at www.epa.gov onday/mahaffey.pdf.
Jane Houlihan of the Environmental Working Group said the study showed the government needs to limit emissions by coal-burning power plants, which are the top source of mercury contamination in the United States.
Her group called for the Food and Drug Administration to issue a list of fish that are lower in mercury and thus safer for pregnant women to eat, such as wild salmon and haddock.
The EPA says the most contaminated fish include shark, tilefish, king mackerel and swordfish.
Sources of healthy omega-3 fatty acids other than fish include walnuts and flaxseed oil, and some fortified foods.