PLACENTA EXTRACHORIALIS

Placenta extrachorialis is a morphological abnormality of the placenta, defined as "a condition in which the transition from a membranous to villous chorion does not occur at the placental edge but at some variable distance within the circumference of the placenta" (1).

Therefore there is placental tissue beyond the limits of the chorionic plate. The basal plate of villous tissue (located on the maternal side of the placenta) extends laterally beyond the chorionic plate from which the villi originate (located on the fetal side).

The chorionic plate is thus smaller than the basal plate (1). This arrangement leaves some placental tissue outside the limits of the chorionic plate, hence the name placenta extrachorialis (1,2).

There are two types of extrachorial placentas:

1. Circummarginate Placenta. The margin of the chorionic plate appears as a raised, thin fibrous rim at the margin of the chorionic plate where the fetal vessels appear to terminate.
2. Circumvallate Placenta. A thickened membranous rim, composed of a double fold of amnion and chorion with degenerated decidua and fibrin in between, is folded inward towards the center.

REFERENCES

1. Scott JS. Placenta extrachorialis (placenta marginate and circumvallate). J Obstet Gynecol Br Emp 1960;67:904-918.
2. Torpin R. Evolution of placenta circumvallate. Obstet Gynecol 1966;27:98-101.