While relatively uncommon, triple-negative breast cancer is a particularly aggressive subtype of the disease that does not depend on hormones such as estrogen to grow and spread.
This type of cancer, which accounts for only 10 per cent to 20 per cent of all breast cancers, does not express the genes for estrogen receptor (ER), progesterone receptor (PR) or HER2, and therefore does not respond to hormone-blocking drugs such as Tamoxifen.
"We found that the interval between menarche and age at first live birth is inversely associated with the risk of triple-negative breast cancer," researcher Christopher I Li said.
The Fred Hutchinson Cancer Research Center 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 is also 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 confirmed several previous studies that have suggested that breast-feeding confers a protective effect against triple-negative disease.
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"Breast-feeding is emerging as a potentially strong protective factor against one of the most aggressive forms of breast cancer," Li said in a statement.
The mechanism by which breast-feeding and delaying childbirth reduces the risk of this form of breast cancer is unclear, Li said.
"Our observations that delayed childbearing and breast-feeding are protective against triple-negative breast cancer suggest that variations in reproductive histories by race may to some extent explain the higher rates of triple-negative disease in African-American women," Li said.
The study involved more than 1,960 Seattle-area women between the ages of twenty and forty-four, 1,021 with a history of breast cancer and 941 without.
Reproductive histories among women without a history of breast cancer were compared to those of women with ER-positive (781), triple-negative (180) and HER2-overexpressing (60) breast cancer.
The study was published in the journal Breast Cancer Research and Treatment.