From the 1940s to 1971, diethylstilbestrol (DES), a synthetic form of estrogen, was used as an antiabortive agent, but it wasn't until the 1990s that it was proven to have carcinogenic properties, putting the daughters of women who took it at risk for clear cell adenocarcinoma (Miller, 1999, 1362). By 1953, its antiabortive effects were disproved, yet it wasn't until 1971 that the U.S. Food and Drug Administration (FDA) issued a warning about its side effects. Back in 1989, the initial findings of a historical cohort study of over 6,000 women who had taken DES during pregnancy and were followed for up to 29 years had a total of 198 incidents of breast cancer, which was a moderate increase associated with taking DES (Colton et al, 1993, 2096-2100). The study also suggested that the risk became greater over time, with no increase being found in the first 20 years, but the relative risk between 20 and 25 years later was 1.6, and after 30 years was 2.5. An additional 7 year and 10 month study was continued with this first cohort and a comparison group of unexposed women, as with the first part of the study.
In this later followup, 194 DES women and 147 unexposed women reported breast cancer (Colton et al, 1993, 2098). The cumulative incidence of breast cancer increased more rapidly with time in those women exposed to DES than in those women not exposed to DES. At 20 years, the cumulative incidence of breast cancer was 1.53 percent with DES exposure and 1.29 percent without, and at 30 years it was 3.77 percent with DES exposure and 2.87 percent. In this time, there were 52 deaths from breast cancer in the DES-exposed group and 40 in the unexposed group. These findings showed a moderate but statistically significant increase in rates of breast cancer later in life in women given DES during pregnancy.
A meeting in 1999 confirmed data from several studies showing that the use of DES in pregnant women increased their likeliho...