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 ...