Smooth chorion is formed by gradual degeneration and disappearance of the chorionic villi. Villus chorion maintains its chorionic villi and ultimately gives rise to placenta. Placenta is a lobulated structure which contains cotyledons.
Blood circulates through the intervillous spaces of the cotyledons.
A 13 week fetus. The crown to rump length is 82 cm. At this stage, still the eyelids are fused. A 17 week fetus. Although it appears mature, a fetus at this stage is unable to survive if it aborts spontaneously, as in this case.