Postpartum breast cancer, which is breast cancer diagnosed within 5 to 10 years of childbirth, has poor outcomes compared to breast cancer in age-matched nulliparous or pregnant women, regardless of the breast cancer subtypes, tumor stage or year of diagnosis. In preclinical mouse models, tumor cells show increased proliferation, invasion and metastatic capacities when injected into the involuting mammary gland, compared with the nulliparous mammary gland. Investigations of the microenvironment of the involuting mammary gland that may contribute to tumor promotion reveal increased fibrillar collagen abundance. Collagen abundance and structure regulation is closely associated with breast cancer risk and progression, and here we hypothesize that increased collagen deposition and structure remodeling occurring during weaning-induced involution is similar to tumor associated collagen, referred to as TACS3, which independently predicts poor outcomes in breast cancer patients.