It’s been 42 years since the pesticide dichlorodiphenyltrichloroethane (DDT) was banned in the United States for its harmful effects on wildlife. But scientists are still untangling its connection to human health. A new study published today in The Journal of Clinical Endocrinology & Metabolism suggests that exposure to the compound in utero may increase a woman’s risk of developing breast cancer later in life. The study relies on a unique database of pregnancies in members of the Kaiser Foundation Health Plan in Oakland, California, between 1959 and 1967, when DDT was still used heavily on crops. Women whose mothers had elevated levels of DDT in their blood had a nearly fourfold increase in risk of developing breast cancer by age 52, compared with controls who were matched for a variety of factors, including maternal history of breast cancer. Previous evidence for a breast cancer link has been mixed—one study found increased risk in women exposed before age 14, whereas others found no association—but in a lab dish, DDT has been shown to activate the HER2 gene in human breast cells, which is expressed in some breast cancers. Understanding the risks of DDT is critical, the authors note, because the compound is still used to control mosquitoes and prevent the spread of malaria in Africa and Asia.