The below abnormality is seen in

Ans. A. Monochorionic diamniotic twins"All monochorionic placentas likely share some anastomotic connections. And, with rare exceptions, anastomoses between twins are unique to monochorionic twin placentas.""Artery-to-artery anastomoses are most frequent and are identified 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 superficial vascular connections on the surface of the chorion, deep artery-to-vein communications can extend through the capillary bed of a given villus. These deep arteriovenous anastomoses create a common villous compartment or "third circulation" that has been identified in approximately half of monochorionic twin placentas." "Whether these anastomoses are dangerous to either twin depends on the degree to which they are hemodynamically balanced. In those with significant pressure or flow gradients, a shunt will develop between fetuses. This chronic fetofetal transfusion may result in several clinical syndromes that include twin-twin transfusion syndrome (TTTS), twin anemia polycythemia sequence (TAPS), and acardiac twinning."