A drug that millions of pregnant women took decades ago to prevent miscarriage and complications has put their daughters at higher risk for breast cancer and other health problems that are showing up now, a new federal study finds.

Many of these daughters, now over 40, may not even know of their risk if their mothers never realized or told them they had used the drug, a synthetic estrogen called DES.

The new study suggests that infertility is twice as common and that breast cancer risk is nearly doubled in these daughters.

Debbie Wingard is one of them. The 59-year-old San Diego woman adopted two boys after being unable to conceive and has had breast cancer twice -- when she was 39 and 49.

"There's no knowing what's going to happen as we age. There's always the fear there's going to be another cancer or another outcome," she said. "I don't think I'll ever get to the point where I feel it's behind me."

The sons of DES users also face health risks -- testicular problems and cysts -- but these are less well studied and don't seem to be as common. Even less is known about the third generation -- "DES grandchildren." Some research suggests these girls start menstruating late and have irregular periods, possible signs of fertility issues down the road.

In the United States alone, more than 2 million women and 2 million men are thought to have been exposed to DES while in the womb and may now want to talk with their doctors about when they should be screened for health problems.

"We don't want to cause a panic of everyone rushing out thinking they're going to get cervical or breast cancer. They just need to have that conversation with their physician," said Dr. Sharmila Makhija, women's health chief at the University of Louisville.

The average woman has about a 1 in 50 chance of developing breast cancer by age 55; for DES daughters it's 1 in 25, the study found. The rate of early menopause was about 1 in 50 for the average woman and 1 in 20 for DES daughters.

Rates for other health risks, such as miscarriage and pre-term delivery, varied.

"They've been identified one at a time. Nobody's been able to get the whole picture," said Dr. Robert Hoover, a researcher at the National Cancer Institute.

Dr. Rahel Ghebre, a specialist in women's cancer at the University of Minnesota, said the evidence of harm from DES has been building since the 1970s. "The breast cancer data has not been as solid before," she said. "Now, we're getting a better sense of what those numbers are. They're pretty significant."

At the same time, she said the fact that the risks are still mounting, so long after the drug was discontinued, is striking. "It says a lot about the implications of medications and how much they really impact our system."

Results are in Thursday's New England Journal of Medicine. The study started in 1992 and involved about 4,600 DES daughters and a comparison group of 1,900 women whose mothers had not used DES.