Pregnant women with healthful, balanced diets need no vitamin supplements and can safely gain far more weight than previously believed, according to new guidelines released yesterday by the Institute of Medicine.

The study, written by a panel of medical and nutritional experts and sponsored by the federal government, recommends that women put on between 25 and 35 pounds during a normal pregnancy, a weight gain about 10 pounds greater than previous guidelines suggested.

The report also said that, with few exceptions, commonly used vitamins and other nutritional supplements are of no value during pregnancy, despite the fact that most prenatal programs and many physicians recommend them.

"We just don't see any evidence that all women in prenatal care should have all these vitamin supplements," said Lindsay Allen of the Department of Nutritional Sciences at the University of Connecticut, who headed the institute's committee on nutritional supplements. "By far the majority of women will get the necessary vitamins and minerals they need from their diets."

As many as 95 percent of all pregnant women say they take some kind of supplement during their pregnancy, according to industry estimates.

"It is my understanding that practically all women receiving prenatal care are told to take vitamins," said Janet C. King, head of the Department of Nutritional Sciences at the University of California at Berkeley, and another member of the institute panel.

"We looked to see what evidence there was for these recommendations. We could find none, except for iron. Is it necessary? No. Does it accomplish something? Probably not," King said.

Except in cases where women take large quantities of supplements, there was little risk in taking vitamins, King said. The need for vitamins and minerals increases significantly when women are pregnant, but the panel maintains that almost all of these additional requirements can be meet by eating a balanced diet.

A trade association of vitamin makers immediately attacked the report. "This outrageously anti-health report potentially dooms thousands of children to an early death or serious birth defects," said J.B. Cordaro, president of the Council for Responsible Nutrition, which represents about 60 manufacturers of vitamins, minerals and other food supplements.

Adrianne Bendich, a medical spokesman for Hoffman-LaRoche, makers of vitamins, said in a statement, that "many women -- especially those with the riskiest pregnancies -- will not have the resources, knowledge, discipline, awareness, and/or inclination to eat well. To frighten such women away from a nutritionally balanced, economical and safe multivitamin supplement is simply irresponsible."

"This is a big market for vitamin makers," King said. Cordaro estimated the U.S. vitamin market at about $3 billion a year. He said he did not know how much of the market was pregnant women. Unlike drugs, vitamins and other food supplements do not have to be proven effective in order to be sold.

Cordaro said that one supplement, folic acid, has been shown to be important in preventing neural tube defects, such as spina bifida. However, the Institute of Medicine panel found that there was insufficient evidence to recommend that all women take supplements containing folate, which is found in cereals, fruits, legumes and vegetables.

The institute report was sponsored by the Department of Health and Human Services, and will play a role in decisions on federal subsidies and support for vitamin use during pregnancy. The Institute of Medicine advises the federal government on health matters.

In addition to its findings on vitamins, the panel suggested that women overall gain more weight than previously recommended, in hopes of reducing the risk of delivering a baby with a low birthweight.

The last study by the institute, done more than two decades ago, recommended a gain of 20 to 24 pounds.

Rather than setting a single target weight, the new guidelines are for women with normal pregnancies to gain between 25 and 35 pounds. Thin women should try to gain between 28 and 40 pounds; obese women should gain about 15 pounds. For most women, the gain should be steady -- about a pound a week in the second and third trimesters.

Because teenagers and black women often have smaller babies, the panel suggested that they too try to gain more weight while pregnant.

While the new guidelines state that for most women vitamins are unnecessary, the report does recommend supplements for some women. The panel concluded that additional vitamins and minerals may be appropriate for strict vegetarians, smokers, women pregnant with more than one baby and women who use illegal drugs or who drink heavily. Recent immigrants and poor women also should consider supplements, the panel said.

Iron was the only nutrient that the panel believed would be inadequate in most diets, so the group suggested that pregnant women take a low-dose supplement of 30 milligrams of ferrous iron daily, which is the same amount as the recommended dietary allowance.

The panel, however, cautioned that there was a danger of different nutrients interacting in adverse ways. Iron, for example, inhibits the absorption of zinc, another common dietary supplement, while zinc inhibits the absorption of copper.

Large quantities of nutrients, so-called megadoses, can also be harmful, according to the panel. The group also discouraged pregnant women from using special high-protein powders and beverages, maintaining instead that women should increase their protein by eating more meat and dairy products.