Rather than villous enlargement, placentomegaly often may result from collections of blood or ibrin, which impart heterogeneity to the placenta. Examples of this are discussed on page 114 and include massive perivillous fibrin deposition, intervillous or subchorionic thromboses, and large retroplacental hematomas.
he chorionic plate normally extends to the periphery of the placenta and has a diameter similar to that of the basal plate. With extrachorial placentation, however, the chorionic plate fails to extend to this periphery and leads to a chorionic plate that is smaller than the basal plate (Fig. 6-2). Circummarginate and circumvallate placentas are the two types. In a circummarginate placenta, ibrin and old hemorrhage lie between the placenta and the overlying sheer amniochorion. In contrast, with a circumvallate placenta, the chorion periphery is a thickened, opaque, gray-white circular ridge composed of a double fold of chorion and amnion. Sonographically, the double fold can be seen as a thick, linear band of echoes extending from one placental edge to the other. On cross section, however, it appears as two "shelves," with each lying above an opposing placental margin (see Fig. 6-2). his anatomy can help diferentiate this shelf from amnionic bands and amnionic sheets, which are described on page 116.
In relatively small observational studies of circumvallate placenta diagnosed postpartum, it was associated with increased risk for antepartum bleeding, abruption, fetal demise, and preterm birth (Lademacher, 1981; Suzuki, 2008; Taniguchi, 2014). In a prospective sonographic investigation of 17 cases, however, Shen and associates (2007a) found most circumvallate placentas to be transient. Persistent cases were benign. In general, most otherwise uncomplicated pregnancies with either type of extrachorial placentation have normal outcomes, and no increased surveillance is usually required.
Functionally, placental perfusion disorders can be grouped into: (1) those in which maternal blood low to or within the intervillous space is disrupted, and (2) those with disturbed fetal blood flow through the villi. These lesions are frequently identiied in the normal, mature placenta. Although they can limit maximal placental blood flow, functional reserve within the placental prevents harm in most cases. Indeed, some estimate that up to 30 percent of placental villi can be lost without untoward fetal efects (Fox, 2007). If extensive, however, these lesions can profoundly limit fetal growth.
Lesions that disrupt perfusion are frequently seen grossly or sonographically, whereas smaller lesions are seen only histologically. With sonography, many of these, such as subchorionic fibrin deposition, perivillous ibrin deposition, and intervillous thrombosis, appear as focal sonolucencies within the placenta. Importantly, in the absence of maternal or fetal
FIGURE 6-2 A. In this illustration, circummarginate (left) and circumvallate (right) varieties of extrachorial placentation are shown.
Double fold of amnion & chorion
A circummarginate placenta is covered by a single layer of amniochorion. B. This transabdominal gray-scale sonographic image shows a circumvallate placenta. The double fold of amnion and chorion creates a broad, opaque white ring and ridge on the fetal surface.
complications, isolated placental sonolucencies are considered incidental indings.
These collections are caused by slowing of maternal blood low within the intervillous space. In the portion of this space near the chorionic plate, blood stasis is prominent and leads to subsequent ibrin deposition. In viewing the placental fetal surface, subchorionic lesions are commonly seen as white or yellow, firm, round, elevated plaques just beneath the chorionic plate.