A 54-year study from the Endocrine Society is the first to directly link breast cancer risk to in utero exposure to the chemical pesticide DDT.

The study, conducted by the Public Health Institute in Berkeley, California, found women exposed to high levels of DDT before they were born had nearly four times the risk of developing breast cancer as an adult.

According to the United States Environmental Protection Agency, DDT is classified as a probable human carcinogen.  The chemical is known to be very persistent in the environment, accumulates in fatty tissues, and can travel long distances in the upper atmosphere. Although DDT (dichloro-diphenyl-trichloroethane) was banned by many countries in the 1970s, including the United States in 1972, many women were exposed in utero to the pesticide in the 1960s when it was still being widely used. Now, those women are reaching the age range of heightened breast cancer risk.

Dr. Barbara A. Cohn, study author and director of the Child Health and Development Studies (CHDS) at the Public Health Institute told FoxNews.com that the women studied didn’t have any special connections or life circumstances that would subject them to out-of-the-ordinary or more than “everyday living-” type exposure to DDT.

“The mothers in our research population, were not special with respect to DDT exposure,” Cohn said. “Use of DDT peaked in 1959 and was still in active use in the U.S. in the 1960s.  This is likely why all women we tested had DDT in their blood.”

The study tracked the daughters of women who participated in the Child Health and Development Studies (CHDS) for 54 years beginning in utero. According to study authors, CHDS participants were members of the Kaiser Foundation Health Plan who gave birth in Oakland, California or nearby. They enrolled in the study during their pregnancies during 1959-1967 and gave blood samples at that time that measured DDT levels during pregnancy and in the days immediately following delivery. Approximately 20,754 pregnancies were studied, giving birth to 9,300 daughters during that period.

For the current analysis, researchers collected data from the participants’ grown daughters to determine how many were diagnosed with breast cancer by age 52. Findings showed that independent of the mother's history of breast cancer, elevated levels of o,p'-DDT, a more estrogenic form of DDT found in commercial DDT,  in the mother's blood was associated with a nearly four-fold increase in the daughter's risk of breast cancer.

The specific type of the chemical, o,p'-DDT is significant in the study because it was among the first recognized endocrine disruptors, shown to mimic and interfere with the function of estrogen hormones.  Because of this, DDT exposure is linked to birth defects and reduced fertility. Among the women diagnosed with breast cancer in the Endocrine Society study, 83 percent had estrogen-receptor positive breast cancer, a form of cancer that receives signals from the hormone estrogen to promote tumor growth.

Researchers also determined that exposure to higher levels of o,p'-DDT was associated with women being diagnosed with a more advanced stage of cancer, as well as being more likely to develop HER2-positive breast cancer, a type which overproduces a specific protein. HER2-positive breast cancers tend to grow faster and are more likely to spread and recur compared to HER2-negative breast cancers.

Cohn said researchers do not have data on precisely how the women were exposed, but have collected information on the various things DDT was used in.

“We do know DDT was in many consumer products at the time, such as bug sprays and wallpaper, that neighborhoods and crops were sprayed, and that DDT was present in food such as milk, butter, fish and meat.  DDT accumulates in animals, particularly in fat and stays in their bodies and also in our bodies for a long time,” she said.

Currently, study authors don’t know why DDT specifically has such a huge impact on breast cancer risk. Cohn said they hypothesize it is a combination of the known interference with hormone function and studies that show the breast may be particularly vulnerable in utero. Next, researchers hope to do experimental studies in animals or with cells and tissue in the laboratory that are better controlled.

“We want to learn whether granddaughters and great-granddaughters could be affected by their grandmother’s exposure to DDT or other chemicals during pregnancy,” she said. “Effects across many generations have been seen for some chemicals in animals.  Our study population, the Child Health and Development Studies, is one of the only possible resources for human research of this kind in our lifetime.  It is a national treasure.”

Cohn said DDT may not be the only culprit, and study authors plan to research the effects of similar chemicals.

“We are currently investigating whether exposure to other environmental chemicals in utero are linked to breast cancer in this same study population. There are many other chemicals that mimic or disrupt hormones,” she said.

The findings are published in the Journal of Clinical Endocrinology and Metabolism (JCEM).