Monochorionic diamniotic twin pregnancies

About 1 in 5 twin pregnancies are monochorionic diamniotic. In these twins, transfusion imbalances may develop because of the vascular anastomoses that are invariably present. The most severe imbalance is twin-twin transfusion syndrome, which presents as a significant amniotic fluid difference diagnosed on ultrasound scan, usually before 26 weeks. Another transfusion imbalance is twin anemia polycythemia sequence, which is a severe hemoglobin difference occurring mainly after 26 weeks in previously uncomplicated pregnancies. Fetoscopic laser coagulation of the anastomoses addresses the cause of these transfusion imbalances. Beside transfusion imbalances, the monochorionic placenta may be unevenly divided, or one part may be dysfunctional, leading to an intertwin growth difference of more than 20%. Here, laser ablation of the anastomoses will usually not cure the problem, but it aims to protect the appropriately growing twin if the growth-restricted cotwin dies. In monochorionic twins, structural anomalies are twice as common as in singleton pregnancies, probably because of a teratogenic effect of the embryonic splitting or because of transfusion imbalances. In contrast, chromosomal anomalies seem less common, possibly because they more likely result in an early demise. Selective reduction is feasible in monochorionic pregnancies but carries increased risks of loss, miscarriage, and early birth than selective reduction in a dichorionic setting. Finally, if one monochorionic twin dies spontaneously, its cotwin may acutely exsanguinate into the demised cotwin, which may result in the loss of both twins or brain damage in the surviving twin. This update addresses the pathophysiology, diagnosis, and management of these common complications in monochorionic diamniotic twin pregnancies.