Cytotrophoblasts (CTBs) differentiate in two directions: syncytiotrophoblasts (STBs) and extravillous trophoblasts (EVTs). STBs are formed by fusion of CTBs on the maternal side. On the other hand, EVTs detach from CTBs in the cell column, and invade the maternal decidua and myometrium. At the decidua, EVTs contact with maternal immune cells. Endovascular EVTs, which replace the endothelium and tunica media in the spiral artery, help maintain maternal blood flow into the placenta. dNK; decidual NK cells, DC; dendritic cells, Mϕ; macrophages.