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Summary: Women whose mothers took the drug DES during pregnancy may have a higher risk for breast cancer than women not exposed to the drug in utero. The new finding suggests these women should be especially vigilant about breast cancer screening, researchers at Boston University School of Public Health say. Their work appears in the journal Cancer Epidemiology, Biomarkers, and Prevention.

Why it's important: DES, or diethylstilbestrol, was commonly given to pregnant women in the 1940s, '50s, and '60s to prevent miscarriage. As many as 2 million women may have been exposed to the drug while still in the womb. Those women are now at the age where breast cancer becomes more common, and thus may be at even higher risk than previously believed.

What's already known: Use of DES was stopped in 1971 after research found that the drug could interfere with the development of the reproductive system of a fetus. Since then, other negative health effects have been identified. Women who took DES while pregnant are known to have a slightly higher risk of breast cancer than women who never used the drug. But it's not clear if daughters exposed to DES in the womb also have an increased risk. However, exposed daughters do have a higher risk of developing a very rare cancer called clear cell adenocarcinoma in the vagina or cervix. They may also have problems getting pregnant and maintaining a healthy pregnancy.

How this study was done: To better understand if women exposed in the womb had a higher breast cancer risk, epidemiology professor Julie Palmer, Sc.D. and her colleagues compared breast cancer risk factors and diagnoses in 4,817 women who had been exposed in the womb and 2,073 similar women whose mothers did not take DES while pregnant. The women had all taken part in previous studies of the health effects of DES. Nearly all of the participants were white. They were followed for 22-24 years.

What was found: There were 76 cases of breast cancer among DES daughters and 26 among unexposed women. After controlling for other risk factors like age, family history of breast cancer, use of oral contraceptives and hormone therapy, number of children, weight and others, the researchers saw differences between the two groups. Women age 40 or older who were exposed to DES in the womb had nearly double the risk of developing breast cancer as unexposed women. Women who were 50 or older had an even higher risk, but there were too few women in the study to determine if that difference was statistically significant. Women younger than 40 did not seem to have any higher risk of breast cancer because of DES exposure. Breast cancer risk seemed to increase more in daughters of women given higher doses of DES.

The bottom line: Although the number of breast cancer cases in the study was small, it provides important information to women whose mothers took DES during pregnancy, said Heather Spencer Feigelson, PhD, MPH, senior epidemiologist at the American Cancer Society.

"These women should continue to be studied," she said. "Most of the women in this cohort are now in their 40s and early 50s, and we do not know whether their breast cancer risk will continue to rise."

Palmer and her colleagues say women whose mothers took DES while pregnant should be especially careful to keep up with their mammograms and clinical breast exams. Breast cancer screening can find the disease at its earliest stages when it is easier to treat. They also say DES daughters should think twice about using postmenopausal hormone therapy.

"Because the commonly used female hormone supplements have been shown to independently increase risk of breast cancer, it might be wise for exposed women to avoid such supplements whenever possible."

Feigelson agrees with that advice. And she says women can take steps to address certain other breast cancer risk factors.

"Engaging in regular physical activity and maintaining a healthy body weight are strategies that all women can employ to reduce their risk of breast cancer," she said.

Citation: "Prenatal Diethylstilbestrol Exposure and Risk of Breast Cancer." Published in the August, 2006 Cancer Epidemiology Biomarkers and Prevention (Vol. 15, No. 8: 1509-1514). First author: Julie R. Palmer, Sc.D., Boston University School of Public Health.