Younger women who postpone childbirth for at least 15 years after their first menstrual period may reduce their risk of developing an aggressive form of breast cancer by up to 60%, according to a study by researchers from the Fred Hutchinson Cancer Research Center in Seattle, Washington.1
The findings show that the interval between menarche and age at first live birth is inversely associated with the risk of triple-negative breast cancer, according to Christopher Li, MD, PhD,a member of the public health science division at Fred Hutch-inson. The study also confirmed several previous studies that con-cluded breastfeeding provides a protective effect against this type of breast cancer. Triple-negative breast cancer accounts for only 10% to 20% of breast cancers and does not express genes for estrogen receptor (ER), progesterone receptor or HER2/neu; therefore, it does not respond to hormone-blocking drugs such as tamoxifen.
Dr. Li and colleagues led the study, which is the first to look at how the interval between first menstrual period and first live birth affects this type of breast cancer. In addition, it is the first to look at the relationship between reproductive factors and breast cancer risk in premenopausal women.
The observational study involved more than 1960 Seattle-area women between the ages of 20 and 44 years; of that group, 1021 had a history of breast cancer whereas 941 did not. Reproductive histories among women without a history of breast cancer were compared with women with ER-positive, triple-negative, and HER2-overexpressing breast cancer.
The study has implications for African American women, who experience disproportionately high rates of the disease for reasons that are largely unknown. However, African American women are more likely to start having children at a younger age and are less likely to breastfeed than non-Hispanic white wom-en, says Dr. Li, noting that variations in reproductive his-tories by race may, to some extent, explain the higher rates of triple-negative disease among African American women.
It is unclear why breastfeeding and delaying childbirth pro-vide protection against triple-negative breast cancer, but previous research has shown that breastfeeding, along with full-term pregnancy, also reduced the risk of ER-positive breast cancer. Researchers believe that is because pregnancy hormones induce certain changes in the breast that make it less susceptible to this type of cancer.
This observational study requires confirmation of results and should be interpreted with some caution, Dr. Li adds.