What is a circumvallate placenta?

A circumvallate placenta is a placenta whose edge is folded back on itself. Normally the membranes that form the baby's sac start right at the edge of the placenta. In a circumvallate placenta, the side facing the baby (the chorionic plate) is smaller than the side attached to the womb (the basal plate), so the membranes fold over and leave a raised ring around the edge.

A 2024 study in the American Journal of Perinatology defines it as "a form of extrachorial placenta in which the basal plate extends past the margins of the chorion plate resulting in the chorion and amnion folding over one another." A Japanese study describes the result as "a raised placental margin in an annular shape". The ring can run all the way round (complete) or only part of the way (partial).

How common is it?

Figures depend on how it is looked for:

Placenta examined after birth: historical pathology studies put it at between 1 in 188 and 1 in 208 placentas. A 2023 meta-analysis of 48 studies gives a combined prevalence of 1.2%, and a Dutch hospital that reviewed 16,042 placental pathology reports found 2.2%.
Seen on a scan in pregnancy: a Texas hospital diagnosed it in 0.20% of mid-pregnancy anatomy scans, at an average of 19.8 weeks.

One reason the scan figure is much lower is that, as the Japanese researchers put it, "the diagnosis is difficult during pregnancy and circumvallate placenta is often detected only by placental examination after delivery".

How is circumvallate placenta diagnosed?

On ultrasound, a sonographer may see a thick line running from one edge of the placenta to the other, or a "shelf" lying over each edge where the membranes fold back.

Scans are not reliable at spotting it. In a 1997 study in which five experienced specialists reviewed focused placental scans, the authors concluded that they "fail to detect the placental edge abnormality in most cases of circumvallation". The same study found that 17 of 49 normal placentas were called probably or definitely circumvallate by at least one reviewer, so false alarms happen too.

The only certain diagnosis is examining the placenta after it is delivered.

Circumvallate placenta risks: what the studies found

Most research looks at placentas examined after birth, and these studies link circumvallate placenta with more complications:

The 2023 meta-analysis found higher rates of emergency caesarean, preterm birth and placental abruption, while stating that "the current evidence is insufficient to draw a robust conclusion".
A Turkish study of 77 pregnancies, confirmed by examining the placenta after birth, found more preterm birth and more babies with growth restriction than in 77 pregnancies without placental problems.
The Japanese study of 92 completely circumvallate placentas at a tertiary perinatal centre reported preterm birth in 64.1%, abruption in 10.9% and neonatal death in 8.9%, alongside more bleeding in early and mid-pregnancy and more waters breaking early.
The Dutch pathology study found circumvallate placenta linked with abruption and infection of the membranes in preterm births, but not in births after 37 weeks.

Why the studies disagree

This is the part most pages leave out. The studies that report poor outcomes mostly identified circumvallate placenta by examining the placenta after birth, and three of them come from tertiary referral centres. They start from a different group of pregnancies than a study of what is seen on a routine scan. The Japanese authors list among their own limitations that their case-control design can make a condition look more common than it is.

The one study built around scan diagnosis found something different. The Texas study compared 179 pregnancies diagnosed on ultrasound with 716 matched pregnancies and found no significant differences in bleeding, preterm birth, abruption, emergency caesarean, stillbirth or babies small for gestational age. Its authors concluded that a prenatal diagnosis "may not warrant additional surveillance during pregnancy".

That study has its own limits. It was retrospective, at one hospital, and depended on doctors with different levels of experience recognising the fold. The honest summary is that the evidence is thin and pulls in two directions.

Does a circumvallate placenta change your care?

Often not. The Texas authors note that "there is currently no change in clinical management following prenatal diagnosis of circumvallate placenta if the pregnancy is otherwise uncomplicated", but that, because of the reported links with small babies and stillbirth, "some consider serial growth assessment". We could not find guidance from NICE, the RCOG or ACOG that sets out specific care for it.

Questions worth asking at your next appointment:

Was the fold complete or partial, and how confident was the sonographer?
Will I be offered growth scans, and if not, why not?
Does anything else in my pregnancy, such as bleeding so far, change the plan?

When to get help

The signs to act on are the same as for any pregnancy, and they are worth knowing well given the reported links with bleeding and early birth.

The NHS says to call 999 if you are pregnant and bleeding and you have severe tummy pain, pain in your shoulder, feel sick, faint or dizzy, or the bleeding is heavy (soaking a pad soon after putting it on). For lighter bleeding or spotting, it says to get urgent help by calling your maternity unit.

The NHS also says to tell your midwife immediately if you notice your baby's movements slowing down or stopping: "Don't wait until the next day." Our page on reduced fetal movements explains what happens when you call.

Frequently asked questions

What causes a circumvallate placenta?

The studies on this page describe how it forms, with the baby's side of the placenta smaller than the side attached to the womb so the membranes fold back, but none establishes why. One Turkish study found smoking was more common in women with a circumvallate placenta, which is an association, not proof of cause.

How does a circumvallate placenta differ from a normal placenta?

In a normal placenta, the membranes of the baby's sac start at the outer edge. In a circumvallate placenta, the membranes fold back on themselves and leave a raised ring around the edge. It can be complete, running all the way round, or partial, and it is often only confirmed when the placenta is examined after birth.

Can you have a healthy baby with a circumvallate placenta?

Yes. In a 2024 study of placentas diagnosed on scan, 179 pregnancies had outcomes similar to matched pregnancies without the finding. Studies of placentas examined after birth report more complications, so ask whether growth scans are planned and keep to the usual advice on bleeding and movements.

Is a circumvallate placenta high risk?

It is not automatically treated as high risk. A 2024 study of scan-diagnosed cases found no increase in complications and suggested extra surveillance may not be needed, while pathology studies report more preterm birth and abruption. Your team will weigh the finding alongside the rest of your pregnancy.

Circumvallate Placenta: What the Evidence Says