When Rachel Carson's "Silent Spring" first sounded the alarm on DDT and its devastating effects on birds and fish, our understanding of how this pesticide affected humans was just beginning. Chemicals can take years to reveal their insidious power, and so for decades, scientists have been piecing together — study by study — the reasons DDT still haunts us today.

First it was breast cancer in women who were exposed to this hormone-disrupting chemical in the 1950s and '60s. Then their daughters, who had been exposed in the womb. Researchers have also linked DDT exposure to obesity, birth defects, reduced fertility and testicular cancer in sons.

Now, a team of toxicologists, molecular biologists and epidemiologists at the University of California, Davis and the Public Health Institute in Oakland have confirmed for the first time that granddaughters of women who were exposed to DDT during pregnancy also suffer from health threats: higher rates of obesity and menstrual periods that start before age 11. Both factors, scientists say, may put these women at greater risk of breast cancer — as well as high blood pressure, diabetes and other diseases.

"This is further evidence that not only is a pregnant woman and her baby vulnerable to the chemicals that she's exposed to — but so is her future grandchild," said Barbara Cohn, director of the Public Health Institute's Child Health and Development Studies, a multigenerational research project that has followed more than 15,000 pregnant women and their families since 1959. "This is something that people had always thought was possible but there had never been a human study to support the existence of that link."

The findings come at a time of renewed interest in DDT, a problem that had been largely tucked into a fading chapter of history. The Los Angeles Times reported last fall that the nation's largest manufacturer of DDT once dumped as many as half a million barrels of its waste into the ocean.

The pesticide, now banned, continues to poison the environment and move up the food chain. As for humans, "there's a clear line you can track of what's happening," said Linda Birnbaum, the former director of the National Institute of Environmental Health Sciences and the National Toxicology Program. "A lot of people want to think that the problems with DDT have gone away, because Congress banned it in 1972. Well, they haven't."

More than 60 years ago, in the heyday of DDT, a team of scientists had the foresight to collect blood samples from more than 15,000 pregnant women in Oakland, Calif. They found, after years of research, that women heavily exposed to DDT during childhood are five times as likely to develop breast cancer, and that a mother's DDT exposure during pregnancy, or immediately after birth, is linked to an increased risk of breast cancer for their daughter. Their daughters are also more likely to experience delays in getting pregnant.

In this most recent study, published in the journal Cancer Epidemiology, Biomarkers & Prevention, researchers found that the risk of obesity in the granddaughters — now in their 20s and 30s — was two to three times greater than women whose grandmothers had little DDT in their blood during pregnancy.

This persistent, generational exposure is likely related to the reproductive system, Cohn said. Since a female is born with all her eggs, a granddaughter is technically also exposed to DDT if her mother was exposed in the womb.

"Even though we banned that stuff more than 40 years ago, people now walking the Earth — the granddaughters of those who were pregnant — were exposed," Cohn said. "It's the full meaning of what a 'forever chemical' is."

Bruce Blumberg, a professor of developmental and cell biology at UC Irvine, still remembers the trucks that used to spray massive amounts of DDT in farms and neighborhoods. At the supermarket where he worked as a kid, he said, "the whole market would be full of a fog of DDT. The industry would want you to believe that chemicals have no effect and … it's all crazy alarmists."

Akilah Shahid said she was shocked, yet fascinated, to learn that she was in the third generation of a study on how chemicals could be affecting women. "How many times have we talked about climate change and things that we need to do better for our children and grandchildren?" she said. "This is more proof that … what we do today is going to affect people way forward."