In a study of 135 mothers and their infants, researchers at Harvard Medical School in Boston found that mothers' fish consumption during pregnancy seemed to aid fetal brain development. Overall, the greater a woman's fish intake during the second trimester, the better her 6-month-old performed on a standard test of mental development.

On the other hand, when mothers had fairly high mercury levels -- as measured in hair samples -- their babies tended to have relatively poorer test scores.

The implication, according to the researchers, is that fish can be a brain-healthy food for women to have during pregnancy -- but only if they eat varieties likely to have little mercury contamination.

Mercury is a metal that occurs in the environment both naturally and as a product of industrial pollution. Most fish and shellfish have some level of mercury, but certain large, long-lived fish are likely to accumulate high levels of the pollutant, which can damage the developing fetal brain.

Because of this, health officials in the U.S. advise pregnant women to avoid certain fish-namely, shark, swordfish, king mackerel and tilefish.

Yet fish also provide a number of important nutrients, including omega-3 fatty acids, which may support fetal brain development.

The new findings, published in the journal Environmental Health Perspectives, point up this potential benefit.

"Women should continue to eat fish during pregnancy but choose varieties with lower mercury contamination," conclude Dr. Emily Oken and her colleagues.

Safer fish choices include canned light tuna, which has less mercury than albacore tuna, and other small fatty fish like salmon, Oken stated. White-meat fish like cod and haddock tend to have low mercury levels-though, she noted, they also have lesser amounts of the beneficial omega-3 fats.

For their study, Oken and her colleagues questioned pregnant women about their fish intake during the second trimester and took hair samples in order to measure the women's mercury levels.

When participants' babies were about 6 months old, the researchers gave them a test that measured their visual memory. The babies were shown a particular picture a number of times so they could become familiar with it. They were then shown that picture along with a second, new picture. The babies' scores were based on how long they gazed at the new picture -- a sign that they remembered the familiar picture and were interested in the new object.

The test is a good way to measure the potential effects of fish intake, Oken said, because it looks at both brain function and vision; omega-3 fatty acids are also essential in eye development.

Overall, babies' scores on the test climbed by 4 points for each weekly serving of fish their mothers had during the second trimester. In contrast, scores went down as mothers' mercury levels rose.

Though health experts advise pregnant women against eating certain fish, they also encourage them to eat up to two seafood meals a week -- a situation, Oken and her colleagues note, that may be confusing to many women. Some, they point out, may only hear that seafood contains mercury and avoid it altogether.

Many U.S. states, Oken said, are now devising clearer recommendations as to which fish are best for women to eat.

SOURCE: Environmental Health Perspectives, October 2005.

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