Drug given to stop miscarriages doubles breast cancer risk for daughters

Aug 8 2006

Scientists in the U.S. have found that the daughters of women who took a common anti-miscarriage drug during pregnancy may be facing twice the the normal risk of developing breast cancer.

Researchers at Boston University have carried out a study of the drug diethylstilboestrol (DES), which was prescribed to pregnant women from the 1940s to 1975, and as a result of their findings are now urging women exposed to the drug in the womb to check their breasts regularly for signs of cancer and not to miss mammograms.

When DES, a synthetic estrogen, was developed in 1938, physicians believed that low levels of estrogen in pregnant women led to spontaneous abortions or premature deliveries.

Although a clinical trial in 1953 indicated no benefit with regard to miscarriage prevention it's use continued in the U.S. until 1971 when researchers determined that DES greatly increased the risk of developing rare cancers of the vagina and cervix in DES daughters; the federal Food and Drug Administration subsequently banned use of the drug in pregnant women.

The scientists found that the danger posed by DES increased with age and while women under 40 did not seem to have any higher risk of breast cancer exposure, those over 40 were twice as likely to develop the disease.

Though women over 50 had an even higher risk, but there were too few in the study to determine whether that difference was statistically significant.

The researchers also say that the greater the amount of the drug taken by the mother, the more likely the daughter would develop the cancer.

For the study the scientists compared rates of the disease in 4,817 women exposed to DES in the womb and 2,073 who were not and identified 76 cases of breast cancer among exposed daughters and 26 among unexposed women.

The drug was given to an estimated 200,000 women in the UK and around 2 million in the US, and Professor Julie Palmer, who led the Boston study, says the news is unwelcome because so many women worldwide were prenatally exposed to DES and they are just now approaching the age at which breast cancer becomes more common.

Previous research had also found that mothers who took the drug faced an increased risk of cancer and the use of DES was stopped in the 1970s after researchers found exposed daughters had a higher risk of developing rare cancers of the vagina and cervix and may also have problems getting pregnant, while exposed sons were found to have suffered low sperm counts and an increased risk of testicular cancer.

Cancer Research UK, says although the study suggests a higher risk of breast cancer for daughters as they get older, it does not mean that they will definitely develop the disease, and early detection is vital.

Palmer says the study may be the first to provide direct evidence that prenatal exposure to excess estrogen may be a risk factor for development of breast cancer.

Although researchers do not know how DES may increase breast cancer risk, some scientists believe the excess estrogen increases the number of breast tissue stem cells available at birth, which could eventually malignantly transform into cancer.

Palmer says if that is the case there is a concern that other environmental factors that increases fetal exposure to estrogenic compounds may do the same thing, and such environmental exposures may deserve more serious consideration.

The study is published in the August issue of the journal Cancer Epidemiology, Biomarkers & Prevention.