Chicago ? Estrogen-progestin pills may cause an aggressive form of breast cancer and make it harder to find tumors until they have reached a later, less-curable stage, according to one of the biggest, most authoritative analyses yet.

The study is part of a run of bad news recently about the hormones routinely taken by millions of women after menopause.

“Hopefully, it will convince women to reconsider,” said Dr. Susan Hendrix of Wayne State University in Detroit, a co-author of the new analysis. “We’ve got to find a better way to help women with their menopausal symptoms.”

Some previous studies suggested tumors might be less aggressive in hormone users; other studies indicated the opposite. Previous research also suggested that hormones might make breast tissue more dense, hindering the detection of tumors.

To try to answer the questions more definitively, the researchers took a closer look at data from the government’s landmark Women’s Health Initiative study, which was halted last summer after it was found that estrogen-progestin pills raise the risk of heart attack, strokes and breast cancer.

While last summer’s findings led many women to stop taking hormones, an estimated 3 million women still use them, primarily to relieve hot flashes and other symptoms of menopause.

The findings appear in today’s Journal of the American Medical Association.

The analysis involved 16,608 women ages 50 to 79 who used either combined hormone treatment or dummy pills for an average of five years.

As of January, breast cancer had developed in 245 women who used the combined hormone treatment and in 185 women who had taken dummy pills.

Hormone users’ tumors were larger at diagnosis, 1.7 centimeters on average versus 1.5 centimeters in placebo women. Tumors had begun to spread in 25.4 percent of hormone users, compared with 16 percent of placebo women.

The researchers said this appears to mean that in women on estrogen-progestin, the tumors both grow faster that is, they are more aggressive and escape detection longer.