Placenta Circumvallata

Definition

Circumvallate placenta is a placental anomaly in which the transition from membranous to villous chorion occurs away from the placental edge, resulting in a central depression surrounded by a thickened, raised, and plicated gray-white ring on the fetal surface of the placenta and a chorionic plate that is smaller than the placental basal plate. The gray-white ring of tissue is composed of a double fold of chorion and amnion, with degenerated decidua and fibrin in between. The ring may be at varying distances from the periphery and may be found around the entire circumference of the placenta or just a portion of it, which is known as a marginal circumvallate placenta. The portion of the placental disk that is not covered by chorion is called the extrachorialis .

Circummarginate placenta refers to a condition similar to circumvallate placenta, but the prominent fold and central depression is lacking. The insertion of the amnion and chorion into the placental disk occurs with a smooth transition and lacks the folding seen in the circumvallate placenta. Again, this area of abnormal insertion of the membranes may be complete or limited to a portion of the placenta. Some placentas may show combined elements of circummarginate placenta and circumvallate placenta.

Prevalence and Epidemiology

The reported prevalence of circumvallate placenta ranges from 0.5% to 21%. The wide variation in prevalence is caused by multiple factors. There are no uniform inclusion criteria, so cases included may be complete and/or partial circumvallate. Population and temporal variation also accounts for some of the wide variation. More recent studies have placed the prevalence of circumvallate placenta between 1% to 7% of deliveries.

Definite risk factors have not been identified. A few reports suggest a higher risk in a multigravida and a risk of recurrence in future pregnancies.

Etiology, Pathophysiology, and Embryology

The cause of circumvallate placenta is unclear, and many theories have been proposed. One theory suggests that the extrachorialis develops as the result of bleeding at the edge of the placenta early in pregnancy. Other theories attribute this condition to abnormal implantation. Older explanations theorized that the development of circumvallate placenta is caused by shallow placental implantation into the decidual layer of the endometrium. The most widely held belief currently suggests that this condition results from excessive implantation of the blastocyst into the endometrium. According to this deep implantation theory, an early placenta covers more than half of the fetal sac as a result of the deep implantation, and as the amniotic sac expands, the peripheral excess placental tissue is withdrawn from the uterine wall. The detached area of placenta slowly atrophies, leaving a double fold of chorion and amnion, with degenerated decidua and fibrin in between. This fold then settles away from the edge of the placenta, giving the characteristic rim of tissue seen on visual inspection. The ring itself may extend through the whole circumference of the placenta, or just a portion of it. The fetal surface within the ring appears normal except that large vessels terminate abruptly at the ring edge. The final result is an area, either all or partway around the placenta, where some of the placental tissue is not covered by the chorionic plate.