Study: Breast Cancer Risk Remains Decades After Giving Birth

A study published in the Annals of Internal Medicine found that compared with women who have never had children, women who have given birth may have an increased breast cancer risk for more than 20 years after the last birth. The risk of breast cancer appeared to peak 4.6 years after birth (hazard ratio [HR] = 1.80; 95% CI, 1.63-1.99), and breastfeeding did not appear to weaken the risk.  

Researchers assessed individual-level data from 15 prospective, international cohort studies, which were part of the Premenopausal Breast Cancer Collaborative Group. These studies included women aged <55 years who did not have breast cancer at enrollment and were followed through direct contact or via cancer registries. Among the 889,944 women (mean age at entry = 42 years) in the study, approximately 18,800 cases of breast cancer were diagnosed before age 55 years. About 720,000 women were parous at enrollment, and about 72,000 contributed one or more births during follow-up.  

The risk of breast cancer decreased to its lowest point at 34 years after birth (HR = 0.77; 95% CI, 0.67-0.88), and the association crossed over from positive to negative at about 24 years after birth. 

The overall pattern was driven by estrogen receptor (ER)–positive breast cancer (accounting for 76% of breast cancer cases), but no crossover was observed for ER-negative cancer. The risk for ER-negative breast cancer was highest about two years after birth (HR = 1.77; 95% CI, 1.34-2.33) and began to decline at about 34 years after birth (HR = 1.38; 95% CI, 101-1.88).  

Family history of breast cancer increased this breast cancer risk. Other factors associated with a higher breast cancer risk included a first full-term pregnancy at an older age (older than 30 years), recent birth (within about 10 years), and taking diethylstilbestrol during pregnancy. 

Despite these findings, experts note that the risk of breast cancer related to pregnancy remains small. 

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