The most common malignancies associated with pregnancy are pregnancy-associated breast cancer (PABC), cervical cancer, melanoma, and hematological cancers. Most malignancies diagnosed during pregnancy and/or lactation does not carry an increased risk of mortality, except for patients diagnosed with PABC or with ovarian cancer during lactation. Placental metastasis almost always occurs in the presence of widespread hematogenous dissemination of disease and usually indicates poor maternal outcome. Similarly, fetuses that develop maternally derived metastases have an exceptionally poor prognosis, with death typically occurring within three months of diagnosis. During cancer progression, the surrounding microenvironment co-evolves into an activated state through continuous communication with the malignant cells, thereby promoting tumor growth. The author shows that breast cancer cells migrated from the implantation site of the placenta that contained extravillous trophoblast cells (EVT), suggesting it is a non-supportive microenvironment for breast cancer cells.