Women who took the drug DES to prevent miscarriage 20 or more years ago are 40 percent to 50 percent more likely than other women their age to develop breast cancer, a Dartmouth University medical study has found.

Dr. Sidney M. Wolfe, director of the Health Research Group, a branch of Ralph Nader's Public Citizen, said yesterday the figures show that the use of DES has led to about 25,000 more cases of breast cancer than would have resulted without DES.

Wolfe urged Health and Human Services Secretary Margaret M. Heckler to begin an effort to notify the estimated 2 million women who received DES of the increased risk.

Until the early 1970s, diethylstilbesterol (DES) was given to pregnant women in the belief that it prevented miscarriages; it later was shown to be completely ineffective. It also was established that daughters of women who took DES have an increased risk of developing cancer of the cervix or vagina. Several hundred such cases have been documented.

The medical study, published in last week's New England Journal of Medicine, compared a group of nearly 2,700 women who had taken DES between 1940 and 1960 with a group of otherwise similar women who were pregnant during that time.

Of the DES women, 4.40 percent had breast cancer, compared with 3.12 percent of the control group. When the figures were adjusted to eliminate potentially confounding factors, the researchers said, the DES group's breast-cancer rate was 47 percent higher.

"DES thus becomes the major environmentally identified cause of breast cancer," Wolfe said. "All doctors who have ever administered DES to women should search their files to identify all such women and immediately notify them."

The study was conducted by Dr. E.R. Greenberg of Dartmouth University medical school and included women from New Hampshire, Massachusetts and Minnesota.