Hemorrhage following partial or complete separation of the placenta can be torrential. Recall that the amount of blood flowing through the intervillous space at term exceeds 600 mL/min (Pates, 2010). In the second half of pregnancy, three placental disorders contribute substantially to maternal mortality rates. These include placental abruption, placenta previa, and placenta accreta spectrum.
The contributions of hemorrhagic placental disorders to maternal mortality are discussed in Chapter 1 (p. 7) and Chapter 42 (p. 733). The management and clinical experience with these disorders span more than a generation. Placental abruption, for example, has been emphasized in this text for more than 50 years beginning with the work of Dr. Jack Pritchard. Now more common than years past, placenta accreta spectrum is another substantial threat to maternal well-being.
Separation of the placenta—either partially or totally—from its implantation site before delivery is called placental abruption or abruptio placentae. From Latin, the latter translates as “rending asunder of the placenta,” which denotes a sudden accident, which is characteristic of most cases. In the purest sense, the cumbersome—and thus seldom used—term premature separation of the normally implanted placenta is most descriptive because it excludes separation of a placenta previa.
Abruption likely begins with rupture of a decidual spiral artery and hemorrhage into the decidual basalis. The subsequent expanding retroplacental hematoma splits the decidua and leaves a thin layer adherent to the myometrium. The decidual hematoma grows to lift away and compress the adjacent placenta. In some cases that are associated with preeclampsia, impaired trophoblastic invasion with subsequent atherosis is one underlying predisposition (Brosens, 2011). Inflammation or infection also may be contributory (Mhatre, 2016). However, histological findings cannot be used to determine the timing of the abruption (Chen, 2017).
In the early stages of placental abruption, clinical symptoms may be absent. Even with continued bleeding and placental separation, placental abruption can still be either total or partial (Fig. 43-1). With either, bleeding typically insinuates itself between the membranes and uterine wall, ultimately escaping through the cervix to cause external hemorrhage. Less often, the blood is retained, leading to concealed hemorrhage and delayed diagnosis. The delay translates into greater maternal and fetal hazards. With concealed hemorrhage, the likelihood of consumptive coagulopathy is also increased. This is because increased pressure within the intervillous space, caused by the expanding retroplacental clot, forces more placental thromboplastin into the maternal circulation (Chap. 44, p. 775).
Schematic of placental abruption. Shown to the left is total placental abruption with concealed hemorrhage. To the right is a partial abruption with blood and clots that dissect between membranes and decidua to reach the internal cervical os and then the vagina.