One third of twins are monochorionic, yet they account for 75% of complications in twin pregnancies. These complications include twin to twin transfusion syndrome, selective intrauterine growth restriction, twin anemia polycythemia syndrome, twin reversed perfusion syndrome, discordant anomalies and single twin demise. Frequent ultrasound surveillance is used to monitor these pregnancies. The estimated date of delivery and confirmation of the amnionicity and chorionicity should occur between 7 -14 weeks. Thereafter, ultrasound surveillance should occur every 2 weeks beginning at 16 weeks and continue until delivery with assessment of the maximum vertical pocket of each twin, the presence of a fluid filled bladder, and serial fetal growth measurement. Doppler studies of the umbilical artery, ductus venosus and middle cerebral artery may be useful in some cases. This chapter will provide an overview on screening and diagnosis of complications of shared placentation.