Group of cells that differentiates around the periphery of the developing embryo when it is a blastocyst and which will, if all goes well after implantation, form the placenta and membranes.

Stage of development of the early embryo that has undergone blastulation, in which a fluid-filled cavity forms in the formerly solid ball of cells (the morula), about 5 days after fertilisation. For the first time, a distinction can be made between a sheet of cells to one side, which will form the embryo proper, termed the inner cell mass, and the remaining, peripheral cells that constitute the trophectoderm, which -- after the blastocyst "hatches" through the zona pellucida and undergoes implantation -- will form the trophoblast. An old-fashioned term for an inevitable miscarriage, meaning that the ovum (in its classical sense for professional embryologists) has not developed normally after fertilisation, there being present just the supporting tissues and no embryo. The term is descriptive, it has no diagnostic value as to the cause of the miscarriage.

The outermost of the two membranes surrounding the gestational sac that encloses the developing embryo. Separate from the amnion until about 14 weeks of pregnancy, after which the two membranes stick together without intervening fluid. Like the placenta, with which at the margins it is joined seamlessly, derived from trophectoderm.

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.