Placental Masses, Variants, and Placental Lesions

The sonographic appearance of the normal placenta is a homogenously echogenic mass attached to the inner myometrium, that demonstrates evolving echogenicity with advancing gestation. Calcifications at the basal plate are a common finding in the third trimester.¹

The normal shape of the placenta is discoid. Succenturiate and bilobate placenta are variants involving multiple lobes of the placenta. Circumvallate placenta is a variant characterized by a raised edge appearance, with membranes inserting at an inward distance from the edge.

Masses detected within the placenta may be characterized as solid, cystic, or mixed based on their sonographic features. Of solid masses, chorioangiomas are the most common benign neoplasm of the placenta², and are discussed in more detail in separate VISUOG chapters. Vascular lesions, infarcts, placental lakes and molar pregnancy are other placental masses and variants that are likewise discussed in separate chapters. 

This chapter will discuss sonographic features of the normal placenta, other structural variants such as circumvallate placenta or bilobed (succenturiate) placenta and placental masses such as teratomas, cysts, and metastases.

The placenta is the unique organ of pregnancy that develops as the interface between maternal and fetal circulations. It is derived from the embryonic trophoblast, the cells of which invade maternal capillaries to form a space where fetal vessels are bathed by maternal blood. This allows gas and nutrient exchange. The surface opposed to the uterine wall is known as the basal plate, while the surface protruding into the amniotic cavity is called the chorionic plate. Maternal blood supply to the placenta is derived from the terminal branches of the uterine arteries, called the spiral arteries; while fetal blood supply ultimately stems from the umbilical arteries.

Its sonographic appearance evolves over the course of gestation. At the time of the routine mid-trimester anatomy ultrasound, the placenta has a homogenous echotexture, smooth surface, and measures less than 2-3cm in thickness (Nyberg 2003).³

In 1979, Grannum et al. proposed the use of a classification system for placental maturity based on placental texture. A Grade I placenta has scattered echogenicities; Grade II displays echogenic stippling at the basal plate; while a Grade III placenta exhibits echogenic indentations extending between the chorionic plate to the basal plate (Williams OB).⁴ These indentations create discrete components within the placenta. While initially proposed to correlate with fetal pulmonary maturity, in subsequent years, Grade III placentas (i.e. mature-appearing) before term were thought to be harbingers of poor fetal outcome (Schiffer 2021).⁵ More recent literature suggests that the subjectivity of this system limits its effectiveness in clinical use (Sau 2004).⁶

Power Doppler interrogation of the normal placenta demonstrates high-velocity blood flow (“jets”) entering the placenta through the spiral arteries from the basal plate (Hernandez-Andrade 2021).⁷ On the chorionic plate, the umbilical vessels can be seen branching from the cord insertion site using the same technique.

Definition

Cystic lesions within the placenta, measuring <1cm are common and inconsequential.¹⁴ In this article, we discuss well-defined echolucent lesions ≥1cm with or without an echogenic border and lacking blood flow. They may correlate pathologically with decidual septal cyst, intervillous thrombus, or infarction.¹⁵ Placental infarction and intervillous thrombus are discussed in detail separately. Cystic lesions near the surface of the placenta may be described as sub-chorionic or sub-amniotic; where these represent hematoma or other vascular lesions they will be discussed in a separate chapter.

ICD Code

O43.8: Other placental disorders

O43.9: Placental disorder, unspecified

Incidence

Decidual septal cysts are present in 10-20% of placentas from term, uncomplicated pregnancies.¹⁴ Large cysts are less common, but the precise incidence remains unknown.⁹

Pathogenesis

Simple, benign cysts are the result of focal degeneration of cells and/or fibrin.^15,16 Specifically, decidual septal cysts are the result of focal degeneration of cells within decidual septa.¹⁰ Similarly, sub-chorionic and sub-amniotic cysts are thought to represent areas of fibrin degeneration.¹⁷

Pathology

Pathologic correlation for sonographic hypoechoic areas can be difficult. One study demonstrated that only about 50% of hypoechoic areas identified by prenatal ultrasound could be identified by postnatal sonographic examination of the placenta. Of those identifiable postnatally, most were decidual septal cysts or intervillous thrombi.¹⁵

Hypoechoic lesions with an echogenic border (“echogenic cystic lesion”) were shown in one study to correlate with decidual septal cyst in 7% of cases.¹⁸

Associated anomalies

Decidual septal cysts that cause compression of adjacent villi may rarely cause fetal growth restriction. ¹⁴

A study of large hypoechoic lesions within the placenta, notably lacking placental pathological correlation, found that large areas >5cm were associated with features of placental insufficiency such as fetal growth restriction, preeclampsia, and earlier gestational age at delivery.¹⁹

Similarly, where there are many or large cystic areas near the placental surface, there is an association with fetal growth restriction.¹⁶  In particular, where these lesions appear near the placental cord insertion site, they may cause cord constriction, and therefore growth restriction.²⁰

Diagnosis

Hypoechoic areas within the placenta are a common finding, particularly beyond 25 weeks’ gestational age. Cystic lesions should be evaluated for number, position in the placenta, size, border regularity, and vascularity within and to the lesion (i.e. by colour or power doppler).

Definitive diagnosis is made by pathologic examination.

Differential diagnosis

Discrete hypoechoic lesions within the placenta may represent decidual septal cysts, intervillous thrombi, or placental infarction. Other pathologic entities may present with numerous cystic lesions and placentomegaly, such as molar pregnancy and placental mesenchymal dysplasia.² Hypoechoic lesions along the surface of the placenta may represent sub-chorionic or sub-amniotic cyst, hematoma, or focal separation of the amnion and chorion.¹⁶

Implications for sonographic screening

Multiple hypoechoic lesions (≥4 lesions) or large lesions (>3-5 cm) may warrant fetal surveillance for growth and well-being since these features may be associated with placental insufficiency.¹⁴

Prognosis

Large hypoechoic areas in the placenta >5cm in largest diameter, are associated with features of placental insufficiency, such as fetal growth restriction, preeclampsia, and early gestational age at delivery.¹⁹ However, when small, they are benign findings.

Management

As hypoechoic lesions, particularly if multiple (≥4) or large (≥3-5cm), have been associated with growth restriction and preeclampsia, pregnancies demonstrating these features may be monitored with serial ultrasounds for growth, and fetal well-being. The risk of preeclampsia is better characterized by established risk calculators, such as that published by the Fetal Medicine Foundation.²¹

Definition

Circumvallate placenta is a placental variant characterized by a chorionic plate that is smaller than the basal plate. This difference results in complete or partial folding of the membranes back towards the chorionic surface, which may be seen as raised edge.^29,30

ICD Code

O43.1 Malformation of placenta

Incidence

The incidence of circumvallate placenta has been found to range between 0.5 to 18% on postpartum placental examination. Shen et al. found the incidence to be 11.2% in early second-trimester scans between 13-16 weeks’ gestation, however other prenatal studies have reported a wide range from 0.2 – 21%.³¹ The reported incidence depends on careful examination of the full circumference of the placental margin, as well as the gestational age of sonographic examination. In the same study, Shen et al report that the sonographic finding was no longer detected by the mid second trimester.

When postpartum placentas are routinely examined, the incidence of circumvallate placenta has been reported to be up to 6.5% (Wentworth 1968).³²

Pathogenesis

A circumvallate placenta is formed when the chorionic plate of the placenta is smaller than the basal plate. The result is that the membranes appear to insert at an inward distance from the edge of the placenta. It is unclear why this occurs, though it has been theorized that the membranes become tethered due to infarct, hemorrhage, or fibrin deposition, with continued growth of the basal plate and restricted growth of the chorionic plate.³³ 

Shen demonstrated in a study of early second trimester (13-16 weeks’ gestation) that 82% of identified circumvallate placentas appeared normal at follow-up mid-trimester ultrasound, as well as post-natally.³¹ This suggests that the appearance of a circumvallate placenta may resolve with continued growth of the chorionic plate.

Etiology

The exact cause or etiology of the circumvallate placenta is unknown, but some have attributed it to marginal hemorrhage, amniotic fluid pressure, or deep implantation of the placenta.³³

Volodarsky-Perel has also demonstrated a significant association between intramural fibroids and circumvallate placenta, and speculated on the role of excessive trophoblastic invasion or abnormal vascularization as a result of the fibroid.³⁴

Some authors have found a higher rate of smoking in pregnancies with a circumvallate placenta compared to normal placentas, suggesting that this may be a risk factor.^30,33 

Pathology

Examination of the circumvallate placenta post-partum demonstrates a partially or fully circumferential ring of off-white redundant tissue, some distance inward from the placental edge. This ridge of tissue is composed of decidual tissue between layers of chorion and amnion.³⁵ This ridge then is the junction between the smaller chorionic plate and the larger basal plate. Fetal vessels branching from the umbilical cord are not seen superficially to course beyond this ridge.³³

Associated anomalies

Circumvallate placenta is not known to be associated with congenital fetal anomalies. Indeed, exclusion of deformities is an important differentiating feature between circumvallate placenta and amniotic bands.³⁵

Recurrence risk

The recurrence risk of circumvallate placenta is unknown. Previous circumvallate placenta has not been reported to be a risk factor.

Diagnosis

The sonographic appearance of a circumvallate placenta on ultrasound has been described using two criteria³³:

1. An irregular, uplifted edge of the placenta, or
2. Marginal shelf or rim

Shen further describes a placental shelf as a “smooth, regular structure, with its base at the placental edge and tapering towards the free edge”; and found that this was a relatively common finding in the early second trimester, that often resolved by 20 – 22 weeks’ gestation.³¹

The placental shelf is differentiated from amniotic sheets and amniotic bands by only being connected to the placenta, where sheets and bands may not relate to the placental surface.

Differential diagnosis

Amniotic bands

An amniotic band is an avascular structure originating from any point of the amniotic surface. It may or may not be connected to the placental surface. Amniotic bands may be attached to fetal parts, thereby leading to fetal deformations such as limb amputations or facial clefts.³¹ In contrast to circumvallate placenta, an amniotic band does not have a free edge.³⁵ 

Uterine adhesions

Intrauterine adhesions are thought to be secondary to scars or synechiae from previous surgical instrumentation of the uterus, Cesarean delivery, or intrauterine infection.³⁵ In contrast to circumvallate placenta, uterine adhesions may be seen to protrude into or cross the uterine cavity, and are seen to attach to the uterine wall itself.^31,35 They may be covered by layers of chorion and amnion, and may have blood flow demonstrable on Doppler interrogation. They are typically thick compared to thin amniotic bands.

Septate uterus

Septate uterus is a Mullerian anomaly characterized by a protrusion of myometrium from the fundus into the uterine cavity, with a normal outer uterine contour. It may be considered on the differential diagnosis of a tissue protrusion seen on ultrasound. However, it is usually distinguished by its fundal position.³¹

Implications for sonographic diagnosis

Circumvallate placenta has been associated with adverse outcomes such as placental abruption, preterm delivery, small-for-gestational age infants and need for emergency caesarean delivery.³⁶ These associations were described when the diagnosis of circumvallate placenta was made on placental pathology.

However, a more recent cohort study that diagnosed circumvallate placenta sonographically found no correlation with any adverse outcomes.³⁷ The average gestational age at delivery was 39 weeks in this study.

Implications for sonographic screening

The placenta is routinely imaged on prenatal ultrasound after 11 weeks.

The study by Shen et al. noted that while the incidence of circumvallate placenta may be as high as 11% in the early second trimester, however many of these resolve by 20-22 weeks’ gestation.³¹

Since the sonographic diagnosis of circumvallate placenta is not associated with adverse pregnancy outcomes in a large cohort study (179 circumvallate placentas)³⁷, the antenatal finding of a circumvallate placenta need not change routine prenatal care.

Prognosis

There has been debate in the medical literature regarding the significance of circumvallate placenta. Some studies suggest a higher risk of antepartum bleeding, abruption, small-for-gestational age and preterm delivery.³⁶ These findings would be supported by the finding of an association between circumvallate placenta and a higher maternal serum AFP and increased uterine artery Doppler pulsatility index.³⁰

However, other studies do not note an increased rate of adverse outcomes when circumvallate is diagnosed antenatally on ultrasound.³⁷

Management

There is no consensus in the medical literature regarding the risks, and therefore the management of circumvallate placenta. Some suggest increased monitoring for pregnancies in which this placental variant is diagnosed due to some association with adverse pregnancy outcomes. This could include serial ultrasounds for fetal growth and well-being.

Others do not identify an increased risk of adverse outcomes following the sonographic diagnosis of circumvallate placenta, and therefore suggest that increased surveillance is unnecessary.³⁷ Indeed, Shen demonstrated that this finding in the early second trimester often self-resolves by 20-22 weeks’ gestation³¹, again implying that this finding does not require additional ultrasounds.

Definition

Bilobed and succenturiate placenta are variations of placental shape in which there is more than one lobe to the placenta. In bilobate (or bipartite) placenta, there are two equally-sized lobes, separated by an expanse of membranes and connected by placental vessels. In contrast, a succenturiate lobe is a smaller accessory lobe connected to the main body of the placenta by placental vessels.

ICD Code

O43.1 Malformation of placenta

Incidence

In a study of all pregnant individuals having their screening morphology ultrasound at 20 weeks’ gestation in a tertiary care centre (N=6163), the incidence of bilobed or succenturiate placenta was 1.1%.³⁸

In a study of postpartum placentas, the incidence of succenturiate lobe placenta was 0.59%; while that of bilobed placenta was 0.1%. ³⁹

Pathogenesis & Etiology

There are two theories for the pathogenesis of bilobed and succenturiate lobe placentas. The traditional theory is of trophotropism in which the placenta “migrates” toward a source of richer blood supply, and atrophies in areas without. ⁴⁰

Another similar theory posits that the placenta grows in areas of growing vasculature; when there is maternal or fetal vascular pathology, angiogenesis within the placenta is disrupted. This results in the placenta not “filling out” small areas and therefore a bilobate or succenturitate lobe.⁴¹

Pathology

These placental variants are evident on gross pathologic exam as lobes separated by an area of membranes.

Associate Anomalies

The most important association with bilobate or succenturiate lobe is vasa previa. It is not otherwise known to be associated with fetal congenital anomalies.

Recurrent risk

These placental variants have not been demonstrated to have a significant recurrence risk. However, there is some evidence that they are associated with in vitro fertilization, and there may therefore be a risk of recurrence above the baseline population risk in those using this mode of conception. ⁴²

Diagnosis

The diagnosis of bilobate or succenturiate lobe can be made sonographically by identifying a second mass along the uterine wall with the same echotexture as the rest of the placenta. It may sometimes be identified as a band-like structure, with demonstrable blood flow on colour Doppler imaging.⁴³ Such a band would not be attached to the fetus.

Differential diagnosis

The differential diagnosis includes amniotic bands or twin pregnancy. Amniotic bands may be attached to the fetus. They demonstrate blood flow on colour Doppler interrogation.

Twin pregnancy is ideally identified in the first trimester, at which chorionicity is most accurately determined. For those presenting later to ultrasound, a second fetus in addition to the additional lobe of placenta is evident.

Implications for sonographic diagnosis

The most important implication of the sonographic diagnosis of bilobed or succenturiate lobe placenta is the association with vasa previa. Specifically, this condition is known as type II vasa previa, in contrast to vasa previa associated with a velamentous cord insertion (type I vasa previa). Bilobed or succenturiate lobe placentas account for one third of identified cases of vasa previa.⁴⁴

A second lobe of the placenta implies that there are fetal vessels running freely through the membranes. If these lie near or over the internal cervical os (i.e. vasa previa), there is risk of fetal exsanguination with rupture of membranes.

It is recommended to screen for vasa previa using colour Doppler assessment of the internal cervical os when a diagnosis of bilobate or succenturiate lobe placenta is made.⁴⁴

In addition, when a diagnosis of a bilobed or succenturiate lobe placenta is made, care must be taken at delivery that both lobes of the placenta are retrieved.

Implications for sonographic screening

Screening for bilobate or succenturiate lobe placenta has the potential to identify those who might further benefit from screening for vasa previa.

One study of 47 patients with succenturiate lobe placentas compared to 7666 controls found that pregnancies with succenturiate lobes were at higher risk of abnormal fetal heart rate monitoring, postpartum hemorrhage, and retained placenta, in addition to the aforementioned vasa previa.³⁹

Prognosis

The prognosis of bilobed or succenturiate placenta alone is good, with little effect on the fetus unless vasa previa is also diagnosed. Vasa previa requires delivery by Cesarean section prior to term in order to mitigate the risk of fetal exsanguination with rupture of membranes.

Management

When bilobed or succenturiate lobe placenta is diagnosed, care should be taken to rule out vasa previa.

If vasa previa is successfully ruled out, care should be taken at delivery to ensure that both lobes are retrieved at delivery. Otherwise, the retained lobe may be associated with postpartum hemorrhage.

Definition

Teratomas are benign neoplastic lesions arising from totipotent germ cells, and therefore may contain tissues arising from all three germ cell layers.² 

ICD Code

O43.8: Other placental disorders

O43.9: Placental disorder, unspecified

Incidence

Placental teratomas are exceptionally rare. As of 2021, 40 cases have been reported in the literature.⁸

Pathogenesis

There are two theories for the development of placental teratoma. The first postulates that totipotent germ cells from the yolk sac migrate to the placenta in the first trimester.⁹ The second asserts that the “teratoma” is in fact an extreme form of acardiac twinning in a monochorionic twin gestation ⁸

Pathology

Postpartum pathology assessment of placental teratomas demonstrates a mass containing tissue from multiple germ lines, such as skin, hair, teeth or other osseous material, adipose and/or muscle tissue. It is classically located between the amnion and chorion.⁹

Associated anomalies

Placental teratomas are not typically associated with other anomalies, though there are reports of their occurrence in pregnancies complicated by multiple congenital fetal anomalies.^2,9,10

Diagnosis

The sonographic appearance of a placental teratoma is similar to its appearance in other organs (such as the ovary). It is a discrete heterogeneous mass that may contain areas that are cystic, echogenic with acoustic shadowing (i.e. calcifications), or echogenic without shadowing (i.e. fat).² Its blood supply is provided by vessels that are covered only by fetal membranes and without Wharton’s jelly, i.e. not via the umbilical cord.¹¹

Definitive diagnosis is made by postpartum pathologic assessment.

Differential diagnosis

The main differential diagnosis for placental teratoma is fetal acardius amorphous. Fox and Butler-Manuel proposed that the latter could be differentiated from fetal teratoma based on two features: absence of an umbilical cord and/or no evidence of axial development. ⁸

Implications for sonographic screening

Clinicians may choose to serially examine any solid placental lesion for changes in size or sonographic features. Due to the rarity of placental teratoma or malignancy, there is no consensus in the literature on further monitoring. The outcomes for pregnancies with placental teratoma are typically good, and further fetal assessment may not be required.

Prognosis

The prognosis for placental teratoma in case reports is most often favorable, with no other obstetrical or neonatal sequelae.^2,9,12,13

Management

Clinicians may choose to serially monitor a solid placental lesion by ultrasound. Due to the rarity of this condition, there is insufficient evidence in the literature to support a particular surveillance regimen. The placenta should be sent to pathology after delivery. The presence of a placental teratoma is not an indication for Cesarean delivery, which is reserved for typical obstetric indications.

Definition

Metastases within the placenta from maternal malignancy have been reported, although case reports typically describe histologic features of metastases rather than the finding of a sonographically detected mass.^22,23

ICD Code

C79.8: Secondary malignant neoplasm of other specified sites

Incidence

The incidence of cancer in pregnancy is reported to be approximately 1/1500 to 1/1000. Resulting placental metastases are rare, with a 2008 review describing 72 cases in the literature since 1866. ²⁴

Pathogenesis

Metastases to the placenta from maternal malignancy reflects hematogenous spread of malignant cells to the intervillous space, a vascular space that is technically part of maternal circulation. From this space, invasion into fetal trophoblasts and villi has been reported. Conversely, metastases from fetal malignancy is described when fetal villous vessels contain malignant cells; however invasion of the villous stroma itself is rare.²⁴

Pathology

Although breast and cervical cancers are the most common maternal malignancies in pregnancy, the most common malignancies associated with placental metastases are melanoma, lung, and gastric cancers.^25,26 Of fetal malignancies, neuroblastoma is the most commonly associated with placental metastases, followed by hepatoblastoma and leukemia.²³ Metastases from maternal malignancy may have gross placental lesions, although the histologic finding of malignant cells in the intervillous or villous spaces is more common.²⁶ Placentas affected by metastases from fetal malignancy may be bulky and pale, without gross lesions.²⁴

Associated anomalies

Placental metastases may be the result of fetal malignancy, and therefore there may be associated fetal findings of malignancy such as an abdominal mass and/or hydrops or polyhydramnios.^27,28

Diagnosis

Placental metastases may have a variable appearance on ultrasound, and can be isoechoic, hypoechoic, or hyperechoic. They would be expected to demonstrate internal Doppler flow.² However, placental metastases are typically diagnosed on placental pathology.

Differential diagnosis

Placental metastases may be considered when evaluating a solid placental mass with internal Doppler colour flow. Other lesions associated with these findings may include chorioangioma or teratoma.

Implications for sonographic screening

Maternal malignancy with metastases to the placenta has been associated with fetal hydrops, polyhydramnios, and stillbirth. Thus, in cases of known maternal malignancy, the finding of a suspicious placental lesion may prompt serial ultrasounds for fetal growth and well-being in the third trimester.

Prognosis

Metastases to the placenta, whether of maternal or fetal origin, are associated with a poor prognosis. For example, placental metastases of congenital neuroblastoma is associated with high mortality, although there are reports of infants in remission after treatment with chemotherapy when the initial diagnosis was made on placental pathology.²³ In a review of placental metastases arising from maternal malignancies, the median maternal survival was 1 month postpartum and one year infant survival was 51%.²⁶

Management

The management of cancer in pregnancy is beyond the scope of this chapter. In cases of known maternal malignancy and a placental lesion suspicious for metastases, parents may be counselled on the recommended cancer treatment strategy (i.e. surgery, chemotherapy, and/or radiation) and the option for pregnancy termination. In continuing pregnancies, serial ultrasounds can be performed for growth and well-being in the third trimester. In all cases of maternal malignancy or suspected fetal malignancy, the placenta should be sent for pathologic assessment.

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