LAUSR

Key Points Question Is a postpartum breast cancer diagnosis independently associated with risk for breast cancer metastasis or breast cancer–specific mortality? Findings In this cohort study including 2970 patients diagnosed with breast cancer at age 45 years or younger, a breast cancer diagnosis within less than 5 years post partum was associated with elevated risk for metastasis and breast cancer–specific death compared with nulliparous patients with breast cancer, independent of estrogen receptor status or tumor stage. There was increased lymph node involvement and liver metastasis in patients with estrogen receptor–positive disease diagnosed less than 5 years postpartum. Meaning These findings suggest that a postpartum breast cancer diagnosis was an independent risk factor associated with breast cancer progression and implicates postpartum biology in the increased metastasis, including postpartum liver biology.