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According to a study published in the journal Cancer Epidemiology, Biomarkers & Prevention, women exposed to diethylstilbestrol (DES) before birth have an increased risk of developing breast cancer after the age of 40.

Diethylstilbestrol is a synthetic estrogen that was used frequently in pregnant women between the 1940s and 1960s. The drug was used to reduce the risk of miscarriages, though later studies indicated that it most likely had no effect on miscarriage risk.

In 1971 a study reported that girls born to women who had used DES (DES daughters) had a greatly increased risk of developing a certain type of vaginal cancer. The risk of breast cancer in these daughters is also of interest for two reasons: 1) DES has been found to increase breast cancer risk in women who used DES during pregnancy (the mothers); and 2) some researchers have speculated that prenatal exposure to high levels of estrogen-whether naturally occurring or from an estrogen-containing medication-could influence subsequent breast cancer risk.

To gather conclusive evidence about the risk of breast cancer in DES daughters, researchers have had to wait until these women reached the ages at which breast cancer most commonly occurs. In the current study, researchers assessed information from 4,817 DES daughters and a comparison group of 2,073 women who had not been exposed to DES.

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  • Breast cancer developed in 76 of the DES daughters and 26 of the unexposed women. After accounting for other factors that may influence the risk of breast cancer, DES daughters had a 40% increased risk of developing breast cancer.
  • The extent of the increased breast cancer risk varied by age. There was no evidence of an increased risk among DES daughters under the age of 40, but those over the age of 40 had a roughly two-fold increase in risk.

The researchers conclude that prenatal DES exposure appears to increase a woman’s risk of breast cancer after the age of 40. The researchers note that “DES-exposed women should be encouraged to adhere to breast cancer screening guidelines.”

Reference: Palmer JR, Wise LA, Hatch EE et al. Prenatal Diethylstilbestrol Exposure and Risk of Breast Cancer. Cancer Epidemiology Biomarkers and Prevention. 2006;15:1509-1514.

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