Placenta Membranacea

Placenta membranacea is an unusual abnormality of placental form. Although some authors have estimated its occurrence to be 1 in 3300 pregnancies, this approximation is a gross overestimate. Although the entity was described as early as 1807, only a handful of references can be found on placenta membranacea. Many cases are hidden in the literature on placenta accreta. Thus, of the 622 cases of placenta accreta gathered by Fox (1972) for his report, seven were placentas membranacea. Although it is true that some membranaceae are also accretas, it is not the case for all. The placenta membranacea (diffusa) is an organ in which all, or nearly all, of the circumference of the fetal sac is covered by villous tissue. The placental mass is generally thin (1 to 2 cm), and it is often disrupted (Figs. 13.11 and 13.12). Although one may suggest that placenta membranacea is a variant of extensive succenturiate lobe formation, we do not believe it to be the case. There is never a separation or even a hint of segregation of tissue from a main placental disk; the tissue is uniformly thin. This type of placental anomaly has been likened to the placenta found in Suidae, Equidae, and Cetacea. It is not an appropriate comparison, however, because these orders of mammals do not have an invasive placenta. Only a superficial macroscopic similarity to the animals’ placentas exists, and it is thus not appropriate to speak of “atavism.”

Placenta membranacea manifests frequently clinically as early bleeding and placenta previa. Affected pregnancies often also terminate in premature delivery, but not invariably. They also may incur difficulties with placental separation after delivery. The literature on placenta mem-branacea has been reviewed by Finn (1954) and Janovski and Granowitz (1961); later cases were reported by

Figure 13.11. Placenta membranacea at 35 weeks’ gestation. Virtually no free membranes are seen: the placenta is very thin. Patient had recurrent bleeding necessitating hysterotomy.

Mathews (1974), Las Heras et al. (1982), and Hurley and Beischer (1987). A complete review of this unusual anomaly comes from Ekoukou et al. (1995), who found only 36 cases to be reported. More recently, Ahmed and Gilbert-Barness (2003) described three cases without finding an etiologic factor. In fact, the etiology of placenta membranacea is not understood. It is easy to state that those villi that are destined to become the chorion laeve do not atrophy normally and that there is endometrial hypoplasia on “constitutional” or other grounds; but, in general, no author has presented support for these hypotheses. The fact that the anomaly often presents with middle trimester bleeding and is thus often diagnosed with placenta accreta may merely reflect that placenta membranacea also overlies the internal os and thereby has a chance of becoming a placenta previa accreta. Occasional cases, such as the one depicted in the previous edition of this book, may have other areas of accretion. Most reported patients delivered their placentas without evidence of accreta. There has not been any associated fetal growth restriction.

One of the cases, described by Finn (1954), is of particular interest. He was the obstetrician for this “infant” 27 years after her birth. She delivered a macerated, stillborn fetus at 5 months, after several weeks of bleeding. This placenta, just like that associated with her own birth, was a typical placenta membranacea (Finn, personal communication, 1976). Thus, either genetic influences determined

Figure 13.12. Maternal surface of Figure 13.11, showing old clot but the absence of accreta in this placenta membranacea.

This abnormal placenta, or it was the result of an abnormal uterine environment that was under genetic control. It seems highly improbable that such a rare anomaly would occur twice within this one pedigree by accident.

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