The average American woman's risk of being struck by breast cancer sometime in her life jumped from one in 10 to one in nine in 1991, the American Cancer Society says.

The projected increase is the first for breast cancer since 1987, when the estimated risk for the disease rose from one in 11 to one in 10, the society said.Although there is evidence high-fat diets, delayed childbearing, birth control pills and other factors may boost the risk for breast cancer, experts remain uncertain why the danger appears to have steadily increased.

The higher estimate for breast cancer, reported in the society's annual "Cancer Facts & Figures," was based on increased incidence of the disease throughout the 1980s and a longer life expectancy for women, which increases the risk.

Overall, 1.1 million new cases of all types of cancer will be diagnosed among Americans in 1991 and about 514,000 people will die of the disease - about 1,400 each day, the report estimated.

Lung cancer remains the most common and deadliest form of cancer. About 161,000 new cases of lung cancer are expected in 1991 - up from 157,000 in 1990.

About 175,000 women are expected to develop breast cancer in 1991, up from a projected 150,900 in 1990, the society said. About 44,500 women are expected to die from the disease, up from 44,000 in 1990.