The gestational sac begins as a spherical structure, with the fetus surrounded by an amnion, a chorion, and placental villi. One surface of the gestational sac implants into the endometrium and becomes the placenta; the villi on the opposite surface degenerate. When you look at placental slides, you can see the layers of the amnion and chorion, both in the membrane section (Figure 18.1) and on the fetal surface. In both locations, amnion is on the fetal side and chorion on the maternal side. The two membranes can be peeled apart grossly, because there is no tissue connection between the two. As an aside, the inner membrane is also called the amniotic membrane (not amnionic), and the fluid is called amniotic fluid .

The villi are fetal structures; they grow downward from the fetal surface in a branching architecture, like the roots of a tree. Vessels and cells inside the villi are fetal. There should not be any maternal vessels in the placenta itself. The spiral arteries of the decidua (endometrium), invaded and opened by trophoblastic cells, spray maternal blood into the space between the villi.

Immature villi have an open and pale appearance (Figure 18.2); they are large compared with the terminal villi of the full-term placenta (when surface area is most required). They are lined by two cell layers, an outer syncytiotrophoblast and an inner cytotrophoblast layer. Very early villi may have a large heaped-up trophoblastic proliferation on the surface, but it should be polar (only on one surface; for a mental image, google Don King’s hair). Circumferential proliferation is suspicious for hydatidiform mole. Mature villi acquire syncytial knots and perivillous fibrin (like hyaline membranes lining the villi). They are tiny—just large enough to hold a few capillaries (see Figure 18.2).

Twin placentas are divided into categories based on how many cell layers they share. Two separate eggs fertilized by two sperm will always form two separate placentas, although they may mash into each other. With two placentas you will see two chorionic plates and two complete sets of membranes (Figure 18.3); this is called diamnionic-dichorionic (di-di) . An ovum that splits very early can also produce two entirely separate placentas, so a di-di placenta may be either monozygotic or dizygotic twins.

An ovum that splits a little later, after it has already formed a chorion, will produce two separate amnions and two fetuses; this is a diamnionic-monochorionic placenta (di-mo). An even later split produces two fetuses in the same amniotic sac or monoamnionic-monochorionic (mo-mo). If the split occurs any later than this, conjoined twins will develop.