Artery-to-artery anastomoses are most frequent and are identiied on the chorionic surface of the placenta in up to 75 percent of monochorionic twin placentas. Vein-to-vein and artery-to-vein communications are each found in approximately half. One vessel may have several connections, sometimes to both arteries and veins. In contrast to these supericial vascular connections on the surface of the chorion, deep arteryto-vein communications can extend through the capillary bed of a given villus (Fig. 45-19). hese deep arteriovenous anastomoses create a common villous compartment or "third circulation" that has been identiied in approximately half of monochorionic twin placentas.
FIGURE 45-19 Anastomoses between twins may be artery-tovein (AV), artery-to-artery (AA), or vein-to-vein (W). Schematic representation of an AV anastomosis in twin-twin transfusion syndrome that forms a "common villous district" or "third circulation" deep within the villous tissue. Blood from a donor twin may be transferred to a recipient twin through this shared circulation. This transfer leads to a growth-restricted discordant donor twin with markedly reduced amnionic fluid, causing it to be "stuck."
Whether these anastomoses are dangerous to either twin depends on the degree to which they are hemodynamically balanced. In those with signiicant pressure or low gradients, a shunt will develop between fetuses. his chronic fetofetal transfusion may result in several clinical syndromes that include
FIGURE 45-18 Shared placenta from pregnancy complicated by twin-twin transfusion syndrome. The following color code was applied for injection. Left twin: yellow = artery, blue = vein; right twin: red = artery, green = vein. A. Part of the arterial network of the right twin is filled with yellow dye, due to the presence of a small artery-to-artery anastomosis (arrow). B. Close-up of the lower portion of the placenta displays the yellow dye-filled anastomosis. (Reproduced with permission from De Paepe ME, DeKoninck P, Friedman RM: Vascular distribution patterns inmonochorionic twin placentas, Placenta. 2005 Jul;26(6):471-475.)
FIGURE 45-20 These serial sonograms depict an interventricular hemorrhage with parenchymal extension and eventual porencephaly that developed following cotwin demise in a monochorionic pregnancy. From left to right, these images were obtained 1 week,o5 weeks, and 8 weeks following demise of the cotwin.
twin-twin transfusion syndrome (TTTS), twin anemia poycythemia sequence (TAPS), and acardiac twinning.
In this syndrome, blood is transfused from a donor twin to its recipient sibling such that the donor may eventually become anemic and its growth may be restricted. In contrast, the recipient becomes polycythemic and may develop circulatory overload manifest as hydrops. Classically, the donor twin is pale, and its recipient sibling is plethoric. Similarly, one portion of the placenta often appears pale compared with the remainder. The recipient neonate may also have circulatory overload from heart failure and severe hypervolemia and hyperviscosity. Occlusive thrombosis is another concern. Finally, polycythemia in the recipient twin may lead to severe hyperbilirubinemia and kernicterus (Chap. 33, p. 626). he prevalence of TTTS approximates 1 to 3 cases per 10,000 births (Society for Maternal-Fetal Medicine, 2013).