Women who were exposed to the highly toxic chemical dichlorodiphenyltrichloroethane (DDT) while they were still in the womb of their mothers were found to be more at risk to develop breast cancer, according to a new study.

While the use of DDT has been banned in the United States since 1972, researchers from the Public Health Institute (PHI) in California discovered that the pesticide had long term effects on the health of Americans exposed to it, especially to women.

Barbara Cohn, director of PHI's Child Health and Development Studies, and her colleagues examined data collected from the daughters of women who received obstetric care through the Kaiser Foundation Health Plan between the years 1959 and 1967.

The period corresponds to the time when the DDT pesticide was widely used in agricultural settings. It is believed that the chemical accumulated in the fat of livestock animals, and it was detected in milk, cheese, butter and other products produced during the time. DDT was also found in several consumer products in the market such as wallpaper.

Among the 20,000 pregnancies recorded during the period, women gave birth to around 9,300 daughters.

The researchers monitored the DDT levels in the bloodstream of the mothers by taking samples during pregnancy and after they gave birth. They also tracked the development of breast cancer in the women's daughters when they reached 52 years old.

After a 54-year period, they conducted a follow up examination of the DDT levels in the mothers of 118 daughters who had developed cancer of the breast.

The researchers then compared the findings from these women with those taken from 354 women included in the study that did not develop breast cancer.

The study showed that women who were exposed to high levels of DDT while they were in the womb were 3.7 times more likely to develop breast cancer than those who were only mildly exposed to the pesticide.

It also revealed that the more levels of DDT the mother was exposed to during her pregnancy, the higher the risk the daughter would be diagnosed with an advanced form of breast cancer.

The results remained strong despite considering the mother's history of breast cancer, according to the researchers.

A majority of the women who had breast cancer (83 percent) were diagnosed with an estrogen-receptor positive cancer. This suggests that estrogen is needed for the disease to grow.

Earlier studies found that DDT only yielded a weak estrogen-like activity, but Cohn said that she is not aware of any mechanism that could create the connection between the exposure to DDT and the risk of breast cancer.

Shiuan Chen, a cancer biology expert at the City of Hope Cancer Center but was not involved in the study, agreed that establishing the link between increased DDT levels in the blood and the increased breast cancer risk is not possible from the PHI study.

Women born during the time when DDT was still widely used could not have been able to determine whether they were exposed to the pesticide or not, according Cohn and Chen.

Chen urges women to follow the standard recommendations for testing instead, such as undergoing regular mammograms.

The Public Health Institute study is featured in the Journal of Clinical Endocrinology and Metabolism.

Photo: Peter Blanchard | Flickr