The increased risk for breast cancer that occurs after childbirth can last more than 20 years, according to research published online Dec. 11 in the Annals of Internal Medicine.
Hazel B. Nichols, Ph.D., from University of North Carolina at Chapel Hill, and colleagues conducted a pooled analysis of individual-level data from 15 prospective cohort studies to characterize breast cancer risk in relation to recent childbirth.
During 9.6 million person-years of follow-up, 18,826 incident cases of breast cancer were diagnosed. The researchers found that compared with nulliparous women, parous women had a higher risk for breast cancer that peaked approximately five years after birth (hazard ratio, 1.80) and decreased after 34 years (hazard ratio, 0.77). At about 24 years after birth, the association crossed over from positive to negative. This pattern was driven by estrogen receptor (ER)-positive breast cancer, but crossover was not seen for ER-negative breast cancer. When combined with a family history of breast cancer, increases in breast cancer risk after childbirth were pronounced. Risk was greater for women who were older at first birth or who had more births. Overall risk patterns were not affected by breastfeeding.
"Health care providers should consider recent childbirth a risk factor for breast cancer in young women," the authors write.