Rarity of placental metastasis is only apparent, for only few placentas of cancerous mothers have been examined histologically. However, it may show biological and immunological conditions which are characteristics of foeto-placental unit. During metastatic spread of solid tumors or hematologic malignancies in the mother, tumor emboli may be localized in intervillous spaces, without being real placental metastasis. Rarely tumor emboli are able to invade the struma of chorionic villi and produce true placental metastases: twelve such observations have been published, seven of which were malignant melanomas. It is even more exceptional that metastatic spread reaches the foetus. In most of the cases, it is thus protected against maternal cancer. This historical observation holds true. The fear of transplacental graft to the foetus is not an argument favorable of terminating a cancer associated pregnancy and foetal metastasis of maternal origin are not among the causes of congenital cancers in children.