Association of parity and time since last birth with breast cancer prognosis by intrinsic subtype

Background: Parity and time since last birth influence breast cancer risk and vary by intrinsic tumor subtype, but the independent effects of these factors on prognosis have received limited attention. Methods: Study participants were 1,140 invasive breast cancer patients from phases I and II of the population-based Carolina Breast Cancer Study, with tissue blocks available for subtyping using immunohistochemical markers. Breast cancer risk factors, including pregnancy history, were collected via in-person interviews administered shortly after diagnosis. Vital status was determined using the National Death Index. The association of parity and birth recency with breast cancer-specific and overall survival was assessed using Cox proportional hazards models. Results: During follow-up (median = 13.5 years), 450 patients died, 61% due to breast cancer (n = 276). High parity (3+ births) and recent birth (<5 years before diagnosis) were positively associated with breast cancer-specific mortality, independent of age, race, and selected socioeconomic factors [parity, reference = nulliparous, adjusted HR = 1.76; 95% confidence interval (CI) = 1.13-2.73; birth recency, reference = 10+ years, adjusted HR = 1.29; 95% CI, 0.79-2.11]. The associations were stronger among patients with luminal tumors and those surviving longer than 5 years. Conclusions: Parity and recent birth are associated with worse survival among breast cancer patients, particularly among luminal breast cancers and long-term survivors. Impact: The biologic effects of parity and birth recency may extend from etiology to tumor promotion and progression.