Adult daughters of women exposed to the once-common pesticide DDT during pregnancy may have an increased risk for developing breast cancer, a study suggests.  

Researchers followed 9,300 women born from 1959 to 1967, when DDT use was widespread, and found that even after taking their mothers' history of tumors and other factors into account, the daughters had a four-fold increased risk for breast cancer if they were exposed to one form of the insecticide in utero.

“It has long been suspected that environmental chemicals that interfere with hormone systems could be connected to risk of breast cancer; here we found the first direct connection for measured levels of DDT in mothers’ pregnancy blood,” lead study author Barbara Cohn, director of child health and development studies at the Public Health Institute in Berkeley, California, said by email.

Worldwide, breast cancer is the most common malignancy in women. About one in nine women will eventually develop it, according to the National Institutes of Health. The risk increases with age, from 1 in 227 at age 30 to 1 in 26 by age 70. Other factors such as obesity, inactivity, alcohol use or early menstruation can also increase the risk.

The U.S. Environmental Protection Agency lists DDT as a probable carcinogen, but previous research has been mixed about the link to breast cancer.

A form of the pesticide known as o,p’-DDT behaves like the hormone estrogen, which is involved in signaling breast cells to grow and divide. In animal studies, this form of DDT has been linked to a common type of breast cancer that is fueled by estrogen.

In the current study, published in the Journal of Clinical Endocrinology and Metabolism, Cohn and colleagues reviewed blood tests for DDT done in 20,754 women who had babies in Oakland, California when use of the pesticide was widespread. Blood samples were taken several times during pregnancy and one to three days after delivery.

Then, using state cancer registry records, they found 137 daughters who had developed breast tumors by age 52. Some women were excluded because there wasn’t data on the mothers’ DDT exposure or other factors that might influence cancer outcomes such as the mother’s age and weight during pregnancy, race, and whether the daughter was breast-fed.

Researchers found that cancer was more likely among the 103 daughters of women exposed to DDT during pregnancy than among another group of 315 women the same age whose mothers didn’t test positive for the chemical.

Most of the women had tumors fueled by the hormones estrogen (83 percent) and progesterone (76 percent).

Exposure in utero to o,p’-DDT was also associated with more advanced tumors at the time of diagnosis and with a particularly aggressive type of malignancy known as HER2-positive breast cancer.

One limitation of the study is that it only tracked cancer diagnosed by 52, missing any tumors that might develop later in life, when breast malignancies are more likely to occur, the researchers acknowledge.

Even so, the findings add to a growing body of evidence linking DDT and other endocrine-disrupting chemicals to cancer, said Laura Vandenberg, an environmental health researcher at the University of Massachusetts Amherst who wasn’t involved in the current study.

“One major risk factor for breast cancer is lifetime exposure to estrogen,” Vandenberg said by email. “DDT and DDT metabolites – breakdown products produced in our body after DDT exposures – can add up with the estrogen produced by our bodies, as well as other synthetic estrogens in our environment, to contribute to risk.”

While women can’t go back in time and control whether they were exposed to DDT in utero, they can still take steps to reduce their risk of developing breast cancer, said Julia Brody, executive director of the Silent Spring Institute in Newton, Massachusetts, a nonprofit research group focused on environmental factors that influence women’s health.

“One great way to reduce your own breast cancer risk is to stay physically active,” Brody, who wasn’t involved in the study, said by email. “In addition, women can minimize their exposures to a range of chemicals that are hormone disruptors or carcinogens by keeping house dust levels low and choosing products with safer ingredients.”