Since the late 1970s, research has assessed the health risks for women prescribed DES while pregnant. Several studies conducted in the 1970s and 1980s demonstrated a modestly increased risk for breast cancer among DES-exposed women. However, not all findings were statistically significant, meaning that the higher rate of breast cancer among DES-exposed women could have resulted from chance rather than DES exposure. Other studies suggested that higher risks of endometrial (uterine) and ovarian cancer occurred in women prescribed DES while pregnant. Researchers were concerned about DES exposure and the increased risk for all three types of cancer because cancer of the breast, uterus, and cervix are all affected by exposure to hormones.

The researchers designed this study as a follow-up to evaluate long-term cancer risks, especially breast cancer, for women prescribed DES while pregnant. The medical records of women who participated in other research on DES exposure during pregnancy were studied. Information was gathered from 2,019 women prescribed DES while pregnant and 1,978 women who were not prescribed DES while pregnant. In addition, researchers studied the medical records of women from earlier studies who had died and determined how many of those deaths were the result of cancer. Using both sets of medical records, researchers were able to examine the medical records of 3,844 DES-exposed women and 3,716 unexposed women. With this information, the researchers compared the cancer rates of DES-exposed women to the cancer rates of unexposed women; they also compared the rates of cancer for these women in the study to the rates of cancer in the general population of women whose DES exposure status was not known.

The researchers found that women prescribed DES while pregnant had a 20%-30% higher risk of developing breast cancer than unexposed women and women in the general population. The researchers found no increased risk for any other cancers, including endometrial or ovarian cancer.

The findings of this study indicated that approximately 16% of women who were prescribed DES while pregnant developed breast cancer. In comparison, approximately 13% of women who were not prescribed DES while pregnant developed breast cancer. In other words, one in six women exposed to DES while pregnant are likely to develop breast cancer, compared with one in eight women not exposed to DES while pregnant. The increased risk of breast cancer did not appear to be interactive with other risk factors (such as use of hormone replacement therapy (HRT), use of birth control pills, or family history of breast cancer). That means that DES exposure, in addition to HRT or family history, did not increase the risk of breast cancer higher than that caused by DES exposure alone.

Citation: Titus-Ernstoff L, Hatch EE, Hoover RN, Palmer JR, Greenberg ER, et al. Long-term cancer risk in women given diethylstilbestrol (DES) during pregnancy. Br J Cancer 2001;84:125-33.