Parity, breastfeeding, and the risk of early-onset breast cancer in women with a BRCA1 pathogenic variant

Background: It is not clear if breastfeeding and/or parity are associated with the risk of breast cancer among women with a germline pathogenic variant in BRCA1. We sought to evaluate the associations of these two factors with early-onset breast cancer in the BRCA1 pathogenic variant. Methods: This case-control study included individuals with a BRCA1 pathogenic variant enroled in a longitudinal study using reproductive and disease histories ascertained at the time of enrolment. Cases had invasive breast cancer prior to age 45, and controls had no breast cancer prior to age 45. Logistic regression was used to evaluate the associations of parity and breastfeeding with cancer risk. Results: Parity per se was not associated with breast cancer risk (OR = 1.09; 95%CI 0.95–1.25); however, among women who never breastfed, the OR for parous vs. nulliparous women was 1.45 (95%CI 1.20–1.75). After matching for parity, ever breastfeeding was associated with 25% lower odds of breast cancer (95%CI 0.61–0.91), and the odds ratio was 0.53 (95%CI 0.40–0.72) for those who breastfed for 20 or more months. Discussion: Our findings suggest a potential role for breastfeeding in the prevention of young-onset breast cancer among individuals with a BRCA1 pathogenic variant and provide insight into possible prevention targets.