A new study has found that the more times a woman gives birth, the higher her risk of 'triple-negative' breast cancer, a relatively uncommon but particularly aggressive subtype of the disease.
However, women who never give birth have a 40 percent lower risk of such breast cancer, which has a poorer prognosis than other types of breast cancer and does not respond to hormone-blocking therapies such as tamoxifen.
"Unlike most breast cancers, triple-negative tumours don't depend on hormonal exposures to grow and spread, so our assumption going into the study was that reproductive factors would not be associated with a woman's risk of this cancer subtype," said Amanda Phipps, a postdoctoral research associate in the Public Health Sciences Division of Fred Hutchinson Cancer Research Center.
While never giving birth appears to be protective against triple-negative breast cancer, the researchers found that women who remain childless have about a 40 percent higher risk of estrogen-receptor-positive breast cancer - the most common form of the disease.
For the study, which was based on data from the Women's Health Initiative, Phipps and colleagues analysed the detailed reproductive histories of some 150,000 postmenopausal women, more than 300 of whom went on to develop triple-negative breast cancer.
"We do know that the hormones of pregnancy induce certain changes in the cellular structure of the breast. Overall, those changes seem to make the breast less susceptible to cancer. It is possible, however, that the increased risk of triple-negative breast cancer we found in women who had given birth may be due to some abnormal response of their breast tissue to the hormones of pregnancy," said Phipps.
"Another possibility is that pregnancy somehow makes the breast more susceptible to certain carcinogens even while reducing breast cancer risk overall," she said.
"This particular study is significant because it is one of the largest studies ever conducted on the impact of reproductive history on triple-negative breast cancer," she added..
Triple-negative breast cancer, which refers to any breast cancer that does not express the genes for estrogen receptor (ER), progesterone receptor (PR) or Her2/neu, accounts for only 10 percent to 20 percent of all breast cancers, and only in the past decade have researchers become aware that this cancer subtype exists.
"This research reinforces the notion that breast cancer is not just one disease," said Phipps said.
"More research is needed to better understand the causes of the most aggressive and lethal forms of breast cancer. While this study adds to our knowledge base, it should not change women's approaches to breast cancer screening," she added.
The findings are published online ahead of the March 16 issue of the Journal of the National Cancer Institute.