A circumvallate placenta is a variation in placental shape where the fetal membranes fold back on themselves, creating a ring-like structure on the fetal surface. This condition is characterized by an abnormal insertion of the amniotic and chorionic membranes. The structural anomaly results in a reduced functional area of the placenta responsible for nutrient and gas exchange.
Understanding the Placental Structure
A normal placenta has a smooth transition between the chorionic plate on the fetal side and the basal plate on the maternal side, with the membranes inserting at the placental margin. In a circumvallate placenta, the chorionic plate (the surface facing the fetus) is smaller than the basal plate facing the uterus. This size discrepancy causes the amnion and chorion membranes to “double back” and insert inward, away from the actual edge of the placenta.
This inward folding creates a thickened, raised, gray-white ring around the central portion of the placenta. The ring is composed of a double layer of fetal membranes, along with layers of fibrin and degenerated maternal tissue. This structural anomaly restricts the area where blood vessels and villi can effectively exchange oxygen and nutrients. The portion of the placenta outside of this ring is no longer functionally integrated.
How Circumvallate Placenta Is Identified
Circumvallate placenta is most often identified during routine prenatal ultrasound examinations, particularly the second-trimester anatomy scan performed around 20 weeks of gestation. Sonographers look for a characteristic “shelf” or “ridge” of tissue protruding from the placenta’s edge into the amniotic cavity. On a three-dimensional ultrasound, this appearance is sometimes described as the “tire sign” due to the rolled, annular edge it presents.
In many cases, however, the condition is subtle and difficult to definitively diagnose during pregnancy. The diagnosis is frequently confirmed only after delivery when the placenta is physically examined. When an ultrasound suggests the anomaly, it prompts medical professionals to implement a more focused monitoring plan.
Potential Risks to Mother and Fetus
The primary concern associated with a circumvallate placenta is Fetal Growth Restriction (FGR). The reduced functional surface area limits the baby’s access to necessary oxygen and nutrients, leading to slower growth and potentially low birth weight.
Another significant risk is preterm labor and delivery (before 37 weeks of gestation). The abnormal membrane insertion and resulting stress on the placenta are thought to increase the likelihood of premature contractions. Furthermore, there is an increased chance of placental abruption, where the placenta prematurely separates from the uterine wall before birth. This separation can lead to significant bleeding and compromise the fetal oxygen supply.
Oligohydramnios, characterized by abnormally low levels of amniotic fluid, is also frequently observed. This may be related to the placenta’s decreased ability to function optimally or chronic leakage from compromised membranes. Many pregnancies involving a circumvallate placenta, however, progress to term without complication and result in a healthy delivery.
Management and Pregnancy Monitoring
Once a circumvallate placenta is diagnosed or suspected, the management plan focuses on close surveillance to mitigate potential complications. There is no specific treatment to correct the placental shape, so monitoring fetal well-being is the central strategy. Prenatal visits and ultrasound examinations are typically scheduled with increased frequency to track the baby’s growth trajectory and placental blood flow.
Fetal surveillance methods, such as non-stress tests and biophysical profiles, are used to regularly assess the baby’s movement, heart rate reactivity, and amniotic fluid volume. If signs of growth restriction or decreased well-being are noted, the frequency of these tests is increased. The timing and method of delivery are then carefully considered, balancing the risks of premature delivery against the ongoing risks of a compromised intrauterine environment.