Clinical subtypes and prognosis in breast cancer according to parity: a nationwide study in Korean Breast Cancer Society.

We explored the association between parity and the risk of developing a specific subtype of breast cancer. We also assessed the association between parity and prognosis according to subtypes. A total of 158,189 patients were enrolled in the Korean Breast Cancer Society Registry database between 1996 and 2015 in Korea. The database provided information on sex, age, number of parity, surgical method, stage, histological findings, presence of biologic markers, adjuvant therapy, and date and cause of death. The patients with higher parity showed a higher ratio of triple-negative breast cancer (TNBC) and human epidermal growth factor receptor 2 (HER2) subtypes. In univariate analysis, women with TNBC who had more than three children had a worse prognosis compared to other groups (HR 1.83; 95% CI 1.34-2.49; P < 0.001). This association was also observed in women younger than 50 years (HR 1.63; 95% CI 1.07-2.48; P = 0.021). In multivariate analysis stratified by subtypes, women who had more than three children were associated with a worse prognosis in TNBC in the total population (HR 1.53; 95% CI 1.11-2.12; P = 0.011). This association was also observed in patients younger than 50 years of age (HR 1.53; 95% CI 1.09-2.61; P = 0.017). Women who had more than three children were more likely to develop hormone receptor-negative (HR-) subtypes. Women who had more than three children were associated with worse prognosis in patients younger than 50 years of age and in patients with TNBC.