By Kathleen Doheny
TUESDAY, June 16, 2015 (HealthDay News) -- Although the pesticide DDT was banned in the United States in 1972, women exposed to the chemical while they were still in the womb may be more likely to develop breast cancer than women who had less exposure to the pesticide, a new study contends.
"We discovered that daughters who are highly exposed to DDT before birth had four times the chance of breast cancer before the age of 52 than women who were not," said study co-author Barbara Cohn, director of the Child Health and Development Studies at the Public Health Institute in Berkeley, Calif.
"This is a strong effect compared to other modifiable risk factors," Cohn said. For instance, research suggests drinking two alcoholic beverages a day can boost risk by 40 percent, she said.
Although Cohn found an association between pre-birth DDT exposure and later breast cancer, she emphasized that no study of this type can prove cause and effect.
"We can never measure every other single factor" that could affect the outcome, Cohn said. The DDT, she said, could "ride along with another environmental exposure," for instance.
The study, funded by the California Breast Cancer Research Program and the U.S. National Institutes of Health, was published June 16 in the Journal of Clinical Endocrinology & Metabolism.
While DDT is banned in the United States, it's still in use in other parts of the world, Cohn said. In the United States, many women now in their 50s and 60s -- an age when the risk of breast cancer tends to rise -- were likely to be exposed to DDT before birth, she said.
For the study, the researchers tracked more than 20,000 pregnancies among women who were members of the Kaiser Foundation Health Plan from 1959 through 1967. The women gave birth to 9,300 daughters during those years.
The researchers looked at DDT levels in the mother's blood samples during pregnancy or soon after childbirth. Until 2012, they also tracked whether or not breast cancer developed in the daughters of these women by age 52.
During the 54-year follow-up period, the researchers examined DDT levels in mothers of 118 daughters who developed breast cancer. They compared these women to 354 women from the group who didn't develop breast cancer.
Women exposed to the highest levels of DDT in the womb had a 3.7 times higher risk of breast cancer than those who had the lowest exposure to DDT, the study revealed.
And the higher the DDT levels in the mother's blood samples, the more likely a woman was to be diagnosed with more advanced breast cancer, according to the study.
This link held strong even after Cohn's team took into account the mother's history of breast cancer.
Most of the women with breast cancer (83 percent) had estrogen-receptor positive cancer, which is thought to need estrogen to grow. Prior research has suggested that DDT may have a weak estrogen-like activity, according to the study authors. However, Cohn said she doesn't know what the mechanism might be behind the link between DDT exposure and breast cancer risk.
Shiuan Chen, the chair of cancer biology at City of Hope Cancer Center in Duarte, Calif., agreed that explaining the link between high DDT blood levels and the higher risk of breast cancer is not possible from this study.
Women born when the pesticide was still commonly used probably have no way of knowing whether they were exposed, or what levels they were exposed to, Cohn and Chen said.
Chen's advice to women is to follow standard recommendations for screening, such as getting regular routine mammograms. "I don't think we can say more than that" based on this study's findings, he said.
"The best thing any woman can do is talk to her doctor about steps to reduce her risk of breast cancer," Cohn said. The discussion should be based on her known risk factors and medical history.
Cohn plans to follow more generations to see if the breast cancer risk holds, she said. Some animal studies suggest such vulnerability may persist for generations.