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Contrary to popular opinion, pregnant women can get all of the necessary nutrients by eating an adequate, well-balanced diet and do not need vitamin or mineral supplements except for iron, a division of the National Academy of Sciences said Wednesday.

The Institute of Medicine's new guidelines, published in the book "Nutrition During Pregnancy," also said average-sized women should gain 25-35 pounds during pregnancy, up from the group's 1970 recommendation of 20-25 pounds.Underweight women at the start of pregnancy should gain between 28-40 pounds, the institute said, while overweight women should aim for a 15-25 pound weight gain. Obese women should gain at least 15 pounds but seek individual nutritional counseling.

The institute said vitamin and mineral supplements are not only unnecessary but could adversely interact with nutrients obtained through a sufficient well-balanced diet.

"Evidence is not sufficient to conclude that routine supplementation with other nutrients is warranted, although clearly there are situations requiring special consideration," the institute said, noting the exceptions include complete vegetarians, smokers and users of alcohol or illicit drugs.

"Since nutrient supplements typically contain multiple nutrients, there is the potential for nutrient-nutrient interactions during absorption and metabolism. An increase in the concentration of one nutrient may adversely affect the availability, absorption or utilization of other nutrients provided by the supplement and by diet," the institute said.

The institute said iron "is the only known nutrient for which requirements cannot be met reasonably by diet alone" and recommended that during the second and third trimester, pregnant women take 30 milligrams of ferrous iron daily.