PurposeYoung premenopausal women develop breast cancer (BC) within 5–10 years of the last childbirth, known as post-partum breast cancers (PPBC), often present with aggressive disease. The exact mechanisms that lead to poor prognosis in these patients are largely unknown.MethodsWe have evaluated the association of clinical and reproductive factors with BC in a cohort of women ≤ 45 years (N= 155) with long-term follow-up. Based on duration since last childbirth (LCB), grouped patients into PPBC1 (LCB ≤ 5 years), PPBC2 (LCB between 6 and 10 years), PPBC3 (LCB > 10 years), and NPBC (age-matched nulliparous BC patients). We compared disease-free survival and hazard associated with recurrence/metastasis between the groups. RNA sequencing of tumor samples was performed from three parous groups (n= 10), and transcriptomic data were analyzed for differentially expressed genes and altered pathways.ResultsWomen in the PPBC1 group had an early menarche and late age at first and last childbirth compared to other groups. Survival analysis within lymph node-positive tumors showed that PPBC1 tumors had a worse prognosis than PPBC2 and NPBC tumors (p= 0.015 andp= 0.026, respectively). Clustering of the differentially expressed genes between the groups showed distinct expression in early PPBC (E-PPBC) tumors. Pathway analysis revealed upregulation of invasive-related pathways along with T cell exhaustion, extracellular matrix remodeling, angiogenesis, and epithelial-to-mesenchymal transition in E-PPBC tumors.ConclusionEarly PPBC is a unique subtype with aggressive clinical features and distinct biology. Further research is needed to accurately project the risk of recurrence and optimal treatment strategies in these young patients.Supplementary InformationThe online version contains supplementary material available at 10.1007/s10549-023-06956-6.