Placenta circumvallate refers to a placental abnormality in which the membranous chorion transitions to a villous chorion in from the placental edges. The prevalence varies due to a lack of uniform criteria for diagnosis. Bleeding is the most commonly reported side effect, but the majority of pregnancies progress without complications. Serial assessment of fetal growth is recommended when diagnosed prenatally.
Keywordsplacenta, circumvallate, circumvallata
Circumvallate placenta is a placental anomaly in which the membranous chorion transitions to a villous chorion, not at the placental edge, but at an area inward from the placental margins, closer to the insertion of the umbilical cord. This entity was first described in the 18th century. Although previously thought not to affect the course of pregnancy, it may in fact be associated with several adverse pregnancy outcomes.
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.
Manifestations of Disease
The uncovered, extrachorialis layer has been implicated in facilitating chorioamnionitis through the spread of ascending infection and antepartum bleeding caused by lack of amnion-chorion membrane coverage. Both of these potential complications can increase the risk of preterm labor and preterm delivery.
A circumvallate placenta has been associated with various adverse pregnancy outcomes. The range of complications varies between studies as inclusion criteria are not consistent, as discussed earlier. There are studies with no reported differences between patients with and without circumvallate placenta. The majority of pregnancies have no symptoms that suggest the presence of circumvallate placenta. Antepartum hemorrhage is the most common sign and usually the initial complication. Bleeding has been reported in 25%–50% of these pregnancies, thought to be arising from the uncovered, extrachorialis layer being more prone to bleeding caused by the lack of amnion-chorion membrane coverage. A higher risk of placental abruption has also been reported in pregnancies with circumvallate placenta, occurring in approximately 5% of patients with a circumvallate placenta. Bleeding may occur at any gestational age, is usually intermittent, and is variable in amount. The bleeding complications associated with circumvallate placenta can continue after delivery, as it has been associated with postpartum hemorrhage and retained placenta, requiring manual removal of the placenta. Circummarginate placenta has also been associated with bleeding complications in the antepartum period.
A watery vaginal discharge, termed hydrorrhea gravidarum, has been reported in 10% of pregnancies complicated by placenta circumvallate and must be differentiated from rupture of the amniotic membranes. Preterm delivery can occur in 40% of circumvallate pregnancies. Circumvallate placenta may also be responsible for early miscarriages.
Reported neonatal outcomes include intrauterine growth restriction, low birth weight, and oligohydramnios. These complications could potentially be the result of the reported smaller amniotic cavity seen with circumvallate placentas. Other neonatal complications associated with this placental anomaly include low Apgar scores, intrauterine fetal demise, and a higher rate of congenital malformations. Perinatal mortality has been reported in 11% to 33% of pregnancies. Circummarginate placenta has not been associated with increased rates of perinatal death or congenital malformations, but may be associated with an increased rate of preterm delivery.
Imaging Technique and Findings
Although the diagnosis of circumvallate placenta is a clinical one, there are a few reports on ultrasound (US) characteristics of this placental anomaly. US findings may include infolding of the fetal membrane on the fetal surface of the placenta during the middle of the second trimester and a bright border at the periphery of the placenta in the third trimester as a result of fibrin deposits. A shelf of tissue, continuous with the edge of the placenta and protruding into the uterine cavity at the margin of the placenta, has also been described. The extrachorialis may appear to have anechoic subamniotic cystic areas. US may also reveal a detached amniotic membrane, secondary to amnion rupture ( Figs. 101.1 and 101.2 ).