Younger women who wait at least 15 years after their first menstrual period to give birth to their first child may reduce their risk of an aggressive form of breast cancer by up to 60 percent, according to a study from Fred Hutchinson Cancer Research Center, an independent, nonprofit research institute based in Seattle.

The findings, by Dr. Christopher I. Li, a member of the public health sciences division at the Seattle-based clinic, are published online.

"We found that the interval between menarche (the start of menstruation) and age at first live birth is inversely associated with the risk of triple-negative breast cancer," Li says.

While relatively uncommon, triple-negative breast cancer is a particularly aggressive subtype of the disease that doesn't depend on hormones such as estrogen to grow and spread. This type of cancer, which accounts for 10 percent to 20 percent of all breast cancers, doesn't express the genes for what are called estrogen receptor (ER), progesterone receptor (PR), or HER2/neu and therefore doesn't respond to hormone-blocking drugs such as Tamoxifen.

Li and colleagues in the public health sciences and human biology divisions at Fred Hutchinson claim the study is the first to look at how the interval between first menstrual period and age at first birth is related to the risk of this particular type of breast cancer.

It also is touted to be the first study to look at the relationship between reproductive factors and breast cancer risk among premenopausal women, who have a higher risk of triple-negative and HER2-overexpressing breast cancer than postmenopausal women.

The study also confirms several previous studies that have suggested that breast-feeding confers a protective effect against triple-negative disease.

"Breast-feeding is emerging as a potentially strong protective factor against one of the most aggressive forms of breast cancer," Li says.

The mechanism, however, by which breast-feeding and delaying childbirth reduces the risk of this form of breast cancer is unclear, Li says.

Previous research has shown that the risk of the most common subtype of breast cancer, ER positive, is decreased among women who've had a full-term pregnancy and have breast-fed. The reason for this, researchers believe, is that the hormones of pregnancy induce certain changes in the cellular structure of the breast that seem to make the tissue less susceptible to this type of cancer.

The study has particular implications for African-American women, who experience disproportionately high rates of triple-negative disease. While the reason for this remains largely unknown, population-level reproductive characteristics are known to vary by race, and compared to non-Hispanic white women, African-American women are more likely to start having children at a younger age and are less likely to breast-feed, Li says.