Fish can be healthful as well as hazardous, therefore medical experts have grappled for years with what advice to give people, particularly pregnant women, about how much is safe to eat.

A new study by Harvard University doctors concludes that pregnant women can boost their baby’s intelligence by eating fish a couple of times a week, but only if they avoid varieties that contain large concentrations of mercury.

Fish is full of omega-3 fatty acids, which help young brains develop and seem to protect against heart disease. But it also is tainted by mercury, a potent neurotoxin that interferes with the building of brains.

The new study of 135 Boston-area babies is considered important because it quantifies and compares the risks and benefits of a fish diet.

The researchers concluded that pregnant women should eat fish because their babies are likely to score higher on intelligence tests. But they also reported that the benefits of the nutrients disappear and the babies’ intelligence scores drop substantially if the fish contains high levels of mercury.

Nearly all fish contains traces of mercury, but large marine species such as swordfish, shark and albacore tuna accumulate the highest levels.

One in every six people in the United States has mercury levels in excess of the amount that the U.S. Environmental Protection Agency considers safe for a fetus. As a result, about 630,000 babies a year are born with mercury exposure that could reduce their mental abilities, according to EPA estimates.

The women in the study ate fish on average once a week during the second trimester of their pregnancy. The highest intelligence scores were among the babies whose mothers had consumed more than two helpings of fish per week but whose mercury levels remained under 1.2 parts per million, according to the report published online last month in the journal Environmental Health Perspectives.

For each additional serving of fish each week, the babies’ intelligence scores increased by 4 points, or an average of almost 7 percent. But for every increase of 1 part per million of mercury, the babies’ intelligence scores dropped by 7.5 points, or 12.5 percent. A woman could raise her mercury level by 1 part per million if she ate an average-size serving of swordfish once a week, said Dr. Emily Oken of Harvard Medical School, the study’s lead researcher.

The study does not provide details about which fish or how much fish pregnant women should eat. But its findings do support the U.S. Food and Drug Administration’s guidelines.