The disparate impacts of a diagnosis during pregnancy compared with recently postpartum has led to recent recommendations for the separate study of breast cancer during pregnancy and postpartum breast cancer (PPBC).26 In this study with a focus on PPBC, we sought to better understand whether temporal proximity to childbirth was associated with key prognostic features of breast cancer, including tumor stage and estrogen receptor (ER) status, as well as risk of distant metastasis and breast cancer–specific mortality. Here, ESR1 is linked to breast cancer.