Cancer and pregnancy coincide in about one in 1,000 pregnancies. One of the most common malignancies associated with pregnancy is breast cancer. Women with pregnancy-associated breast cancer (PABC) have a higher likelihood of being diagnosed with metastatic disease and estrogen receptor (ER) negative tumors than do non-pregnant women. Controversies exist regarding the effect of pregnancy on breast cancer prognosis. Some researchers suggest that pregnancy does not affect breast cancer prognosis, whereas others claim the opposite. Although PABC is usually discovered in an advanced stage, breast cancer metastasis on the placenta is a rare event. During cancer progression, the surrounding microenvironment co-evolves into an activated state through continuous communication with the malignant cells, thereby promoting tumor growth. The effect of pregnancy and placental environment on breast cancer biology is the issue of this review. Placental and cancer cells implantation processes share similar molecular pathways. This suggests that placental factors may affect breast cancer cells biology. Previously, we analyzed the effect of first trimester human placenta on breast cancer cells. Breast cancer cells were co-cultured with placental explants during their implantation on matrigel substrate. We found that the placenta reduced ER expression on the cancer cells and induced their migration and invasion abilities. As a result of it, breast cancer cells migrated away from the placental implantation sites. Hormonal pathways were involved in these phenomena. These results may explain the high incidence of metastases during pregnancy in on the one hand and the rarity of metastases on the placenta on the other hand.