Twin pregnancies

In dichorionic (DC) twins the inter-twin membrane is composed of a central layer of chorionic tissue sandwiched between two layers of amnion, whereas in monochorionic (MC) twins there is no chorionic layer present.

• The best way to determine chorionicity by ultrasound at 11-13 weeks is to examine the junction between the intertwin membrane and the placenta. In DC twins there is a triangular placental tissue projection (lambda sign) into the base of the membrane.
• 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.