Diagnosis of chorionicity: The role of ultrasound

Chorionicity is the main determinant of the perinatal outcome in twin pregnancies: perinatal mortality and morbidity are significantly higher in monochorionic versus dichorionic twins. This is mainly due to complications associated specifically with monochorionicity, such as twin to twin transfusion syndrome (TTTS), selective fetal growth restriction (FGR) and twin reverse arterial perfusion syndrome (TRAP), consequences of the presence of inter-twin vascular anastomoses in the common placenta. For this reason the diagnosis of chorionicity in twins is of clinical importance in order to plan an increased surveillance in monochorionic gestations and to recognize the appearance of complications in their early stages.