Cells in the developing placenta that are derived from the early embryo's outer layer (the trophectoderm of the blastocyst) and then persist through pregnancy, producing either syncytiotrophoblast or extravillous trophoblast.

The outermost layer of trophoblast, produced by cytotrophoblast where there is direct contact with maternal decidua rather than blood. The EVT cells travel into the decidua, reacting with NK cells and invading maternal blood vessels feeding the placenta, softening the walls and replacing the lining with fetal tissue, a process called "conversion". Also known as interstitial or intermediate trophoblast.

The component of the trophoblast bathed by maternal blood in the placenta (and thus covering the chorionic villi). Derived by differentiation and fusion of cytotrophoblast cells, so the syncytiotrophoblast only ever comprises just one cell (albeit with very many cell nuclei), through which almost everything reaching or leaving the embryo or fetus must pass. By the end of normal pregnancy, its area is about 12 square meters, or over 100 square feet. Most hormones and proteins produced by the placenta, such as progesterone, hCG and PAPP-A, come from it.