In this prospective follow-up study, researchers wanted to learn whether women exposed to DES before birth (in the womb), known as DES Daughters, were more likely to develop breast cancer than unexposed women. Information on reproductive factors, behavioral risk factors and health problems, including breast cancer, was gathered in 1994 from 4,821 DES Daughters and 2,095 unexposed women. Three years later, 3,916 DES Daughters and 1,746 unexposed women completed a short follow-up questionnaire that asked about new occurrences of disease. The researchers confirmed results of the surveys by checking medical reports or death certificates. The data revealed that 43 cases of breast cancer occurred among DES Daughters and 15 cases occurred among the unexposed.

Findings of the study are not definitive, but suggest that exposure to DES before birth (in the womb) may be associated with an increased risk of breast cancer. The risk of breast cancer for DES Daughters as a whole group is slightly higher than estimated in earlier studies (Hatch, 1998), but is not a statistically significant increase (that means the higher incidence of breast cancer could be the result of chance, rather than being associated with exposure to DES).

The study findings were different, depending upon the age of the study participants.

DES Daughters under age 40 did not have a higher risk of breast cancer than unexposed women under 40. However, DES Daughters who were over 40 years old who participated in the study were 2 1/2 times more likely to experience breast cancer than were unexposed women over age 40. The increased risk was statistically significant (not due to chance, but increased risk was more likely to be related to association with DES exposure).

It is important to note that the findings from this study are viewed as preliminary due to some limitations of the research. Although participants had been followed for an average of 19 years, the median age of the study group in 1997 was 43 years, an age when breast cancer incidence is still relatively low in the general population. Continued investigation as the group of DES Daughters grow older is necessary to have enough data to determine if DES exposure is indeed linked to an increased risk of breast cancer among women exposed before birth (in the womb).

In addition, while the DES Daughters and unexposed women in the study were similar in most respects, they differed in some important areas. DES Daughters in the study were more likely to be older when they gave birth to their first child or to have had fewer or no children than were unexposed women. Both these characteristics have been associated with increased incidence of breast cancer and raise questions about whether DES exposure or other characteristics might be responsible for increased risks.

Citation: Palmer JR, Hatch EE, Rosenberg CL, Hartge P, Kaufman RH, Titus-Ernstoff L, et al. Risk of breast cancer in women exposed to diethylstilbestrol in utero: preliminary results (United States). Cancer Causes Control 2002;13:753-8.