Pregnancy-associated breast cancer, which has a poor prognosis, is often overlooked by clinicians and researchers alike. With the trend towards delayed child-bearing, an increase in the occurrence of breast cancer complicated by pregnancy is anticipated. The mechanisms that have been proposed to account for this poor prognosis, including increased hormone exposure, might not contribute significantly to the observed increase in metastasis seen in these patients. Instead, the mammary microenvironment might become tumour-promoting after pregnancy because of the remodelling of the mammary gland to its pre-pregnant state. This remodelling, which is associated with pro-inflammatory and wound-healing mechanisms, is proposed to support tumour-cell dissemination. This hypothesis will be discussed.