Of variants, placentas may infrequently form as separate, nearly equally sized discs. his bilobate placenta may also be called bipartite placenta or placenta duplex. In these, the cord inserts between the two placental lobes-either into a connecting chorionic bridge or into intervening membranes. A placenta containing three or more equivalently sized lobes is rare and termed multilobate. Unlike this equal distribution, one or more
TABLE 6-1. Some Indications for Placental Pathological Examinationa
FIGURE 6-1 Succenturiate lobe. A. Vessels extend from the main placental disc to supply the small round succenturiate lobe located beneath it. (Used with permission from Dr. Jaya George.) B. Sonographic imaging with color Doppler shows the main placental disc implanted posteriorly (asterisk). The succenturiate lobe is located on the anterior uterine wall across the amnionic cavity. Vessels are identified as the long red and blue crossing tubular structures that travel within the membranes to connect these two portions of placenta.
disparately smaller accessory lobes-succenturiate lobes-may develop in the membranes at a distance from the main placenta (Fig. 6-1). hese lobes have vessels that course through the membranes. Of clinical importance, if these vessels overlie the cervix to create a vasa previa, dangerous fetal hemorrhage can follow vessel laceration (p. 118). n accessory lobe can also be retained in the uterus after delivery to cause postpartum uterine atony and hemorrhage or later endometritis.
Rarely, the placental surface area varies from the norm. With placenta membranacea, villi cover all or nearly all the uterine cavity. This may occasionally give rise to serious hemorrhage because of associated placenta previa or accreta (Greenberg, 1991; Pereira, 2013). A ring-shaped placenta may be a variant of placenta membranacea. his placenta is annular, and a partial or complete ring of placental tissue is present. hese abnormalities appear to be associated with a greater likelihood of tion (Faye-Petersen, 2006; Steemers, 1995). With placenta fenestrata, the central portion of a placental disc 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 cotyledon.
During pregnancy, the normal placenta increases its thickness at a rate of approximately 1 mm per week. lthough not measured as a component of routine sonographic evaluation, this thickness typically does not exceed 40 mm (Hoddick, 1985). Placentomegay deines those thicker than 40 mm and commonly results from striking villous enlargement. his may be secondary to maternal diabetes or severe maternal anemia, or to fetal hydrops, anemia, or infection caused by syphilis, toxoplasmosis, parvovirus, or cytomegalovirus. In these conditions, the placenta is homogeneously thickened. Less commonly with placentomegaly, fetal parts are present, but villi are edematous and appear as small placental cysts, such as in cases of partial mole (Chap. 20, p. 391). Cystic vesicles are also seen with placental mesenchymal dysplasia. Vesicles in this rare condition correspond to enlarged stem villi, but unlike molar pregnancy, trophoblast proliferation is not excessive (Woo, 2011).