Chorionicity determination: lambda sign on 3D scan

This video clip illustrates the difference between dichorionic diamniotic (DCDA) and monochorionic diamniotic (MCDA) intertwine membranes. The correct diagnosis of chorionicity is crucial for the management of twins and other multiple gestations. This training video uses dichorionic triamniotic (DCTA) triplet pregnancy as an example of multiple pregnancy placentation types. In dichorionic diamniotic (DCDA) twins the inter-twin membrane is composed of a central layer of chorionic tissue sandwiched between two layers of amnion, whereas in monochorionic diamniotic (MCDA) twins there is no chorionic layer present. MCDA membrane is thin and sometimes it is poorly visible. DCDA membrane is much thicker. The best way to determine chorionicity by ultrasound at 11-13+6 weeks is to examine the junction between the inter-twin membrane and the placenta. In DCDA twins there is a triangular placental tissue projection (lambda sign) into the base of the membrane. Lambda sign (or twin peak sign) is better visible on 3D scan. With advancing gestation there is regression of the chorion laeve and the 'lambda' sign becomes progressively more difficult to identify. Thus by 20 weeks only 85% of DC pregnancies demonstrate the lambda sign. Chorionicity is the main factor determining pregnancy outcome in twins: Miscarriage: In singleton pregnancies with a live fetus demonstrated at the 11-13+6 weeks scan the rate of subsequent miscarriage or fetal death before 24 weeks is about 1%. The rate of fetal loss in DCDA twins is about 2% and in MCDA twins it is about 10%, due to severe early-onset twin-to-twin transfusion syndrome (TTTS). Perinatal mortality: This is about 0.5% in singleton pregnancies, 2% in DCDA twins and 4% in MCDA twins. The increased mortality in twins is mainly due to prematurity-related complications. In MCDA twins in addition to prematurity there are complications from TTTS. Growth restriction: In singleton pregnancies the prevalence of babies with birth weight below the 5th centile is 5%, in DCDA twins it is about 20% and in MCDA twins it is 30%. Early preterm delivery: Almost all babies born before 24 weeks die and almost all born after 32 weeks survive. Delivery between 24 and 32 weeks is associated with a high chance of neonatal death and handicap in the survivors. The risk of spontaneous delivery between 24 and 32 weeks is about 1% in singletons, 5% in DCDA twins and 10% in MCDA twins Structural defects: The prevalence of major defects is about 1% in singletons, 1% in each of DCDA twins and in 4% in each of MCDA twins.