With basic knowledge of placental implantation, development, and anatomy presented in Chapter 3, clinicians can more easily understand the genesis of abnormal placental types. Much of the ever-growing knowledge of placental pathology was stimulated by a nucleus of placental pathologists that includes, among others, Benirschke, Driscoll, Fox, Naeye, Salafia, and Faye-Petersen. For a detailed account of these disorders, the reader is referred to the 5th edition of Pathology of the Human Placenta by Benirschke and colleagues (2006) and the 2nd edition of the Handbook of Placental Pathology by Faye-Petersen and associates (2006).
Abnormal Shape or Implantation
Most placentas are either round or oval, but variations are common. As discussed in this chapter, many of these have clinical importance.
Multiple Placentas with a Single Fetus
Uncommonly, the placenta forms as separate, near equally sized disks. The cord inserts between the two placental lobes—either into a connecting chorionic bridge or into intervening membranes. This condition is termed bilobate placenta, but is also known as bipartite placenta or placenta duplex (Fig. 27-1). Fox and Sebire (2007) reported its incidence to be approximately 1 in 350 deliveries. A placenta containing three or more lobes is rare and termed multilobate.
Bilobate placenta with marginal insertion of the umbilical cord. There also is partial velamentous insertion of the cord with the fetal vessels traversing the membranes to reach the smaller lobe on the right.
These placentas are a smaller version of the bilobate placenta. One or more small accessory lobes develop in the membranes at a distance from the main placenta, to which they usually have vascular connections of fetal origin. Although its incidence has been cited by Benirschke and associates (2006) to be as high as 5 percent, we have encountered these much less frequently. Suzuki and co-workers (2009) noted a twofold higher incidence of succenturiate lobes in twin placentas. The accessory lobe may sometimes be retained in the uterus after delivery and may cause serious hemorrhage. In some cases, an accompanying vasa previa may cause dangerous fetal hemorrhage at delivery.
Rarely, all or a large part of the fetal membranes are covered by functioning villi. Placenta membranacea may occasionally give rise to serious hemorrhage because of associated placenta previa or accreta (Greenberg and colleagues, 1991).
In fewer than 1 in 6000 deliveries, the placenta is annular in shape, and sometimes a complete ring of placental tissue is present. This development may be a variant of placenta membranacea. Because of tissue atrophy in a portion of the ring, a horseshoe shape is more common. These abnormalities appear to be associated with a greater likelihood of antepartum and postpartum bleeding and fetal-growth restriction (Faye-Petersen and colleagues, 2006).
In this rare anomaly, the central portion of a discoidal placenta is missing. In some instances, there is an actual hole in the placenta, but more often, the defect involves only villous tissue, and the chorionic plate remains intact. Clinically, it may erroneously prompt a search for a retained placental lobule.
When the chorionic plate, which is on the fetal side of the placenta, is smaller than the placental basal plate, which is located on the maternal side, the periphery is uncovered, and the term extrachorial placenta is used (Fig. 27-2). If the fetal surface of such a placenta presents a central depression surrounded by a thickened, gray-white ring, it is called a circumvallate placenta. The ring is composed of a double fold of chorion and amnion, with degenerated decidua and fibrin in between. Within the ring, the fetal surface presents the usual appearance, except that the large vessels terminate abruptly at the ring edge. When the ring does not have the central depression, the placenta is described as circummarginate (see Fig. 27-2).
Circumvallate (right) and circummarginate (left) varieties of extrachorial placentas. With circumvallate placenta, the double fold of amnion and chorion creates the broad white ring seen on the fetal surface of these placentas. Circummarginate placentas lack this double fold and white ring.
With circumvallate placentas, there is an increased risk of antepartum hemorrhage—from both placental abruption and fetal hemorrhage—as well as of preterm delivery, perinatal mortality, and congenital malformations (Lademacher and co-workers, 1981; Suzuki, 2008). Adverse clinical outcomes with circummarginate placentas are less well defined.
Placenta Accreta, Increta, and Percreta
These abnormalities are serious variations in which trophoblastic tissues invade the myometrium to varying depths. They are much more likely with placenta previa or with implantation over a prior uterine incision or perforation. Torrential hemorrhage is a frequent complication (see Chap. 35, Placenta Accreta, Increta, and Percreta).
Conceptually, placental perfusion disorders may be grouped into: (1) those that disrupted maternal blood flow to or within the placenta and (2) those that disturb fetal blood flow through the villi. Many of these lesions are common and are found in normal mature placentas. Although they limit maximal placental blood flow, the placenta's functional reserve is great. Some estimate that the placenta can lose up to 30 percent of its villi without untoward fetal effect (Fox and Sebire, 2007). If lesions are extensive, however, they have the potential to profoundly limit fetal growth.
Maternal Blood Flow Disruption
A number of lesions can restrict or abate ...