Early full-term pregnancy is thought to be one of the most effective means of decreasing lifetime breast cancer risk. Paradoxically, young women diagnosed with breast cancer shortly after giving birth have a higher risk of dying from their disease. These seemingly opposing effects have been largely attributed to mammary epithelial stem cell differentiation and precancerous cell proliferation, respectively, induced by pregnancy-associated hormonal changes. However, recent studies suggest that remodeling of the cellular microenvironment and extracellular matrix during pregnancy and involution may contribute to the enhanced invasive and metastatic potential of breast carcinomas and thus lead to their worse clinical outcome.