Most women know the established risk factors for breast cancer such as older age and a family history of the disease, but new research suggests that some less obvious risks are determined early, during a woman's youth and child-bearing years.

Decisions a woman makes then - such as whether to breastfeed or not - may impact her breast cancer risk years in the future, including altering the odds of aggressive disease.

Why does one woman get an aggressive form of breast cancer while another's tumor is easily treated?

Genetics certainly play a role, but new findings suggest factors in a woman's personal history help determine the type of cancer she gets.

Doctors from the Fred Hutchinson Cancer Research Center compared 1,100 women with different types of breast cancer to nearly 1,500 healthy women.

They found that women who breastfed their babies for at least 6 months had a 50% lower risk for so-called triple negative tumors - among the most deadly form of the disease.

They also enjoyed a 20% reduction in a second, less aggressive type of breast cancer.

In contrast, women who began their periods at an early age had more than double the risk for tumors that respond to the drug Herceptin, whereas late onset of menopause was linked to a higher risk for estrogen-sensitive cancers.

Researchers say understanding how a woman's reproductive history alters her risk for certain types of breast cancer should lead to improved screening, diagnosis and treatments.