placental fusion
Placental fusion is a phenomenon that can occur in a twin pregnancy. This can occur to varying degrees. Determination of chorionicity on ultrasound can sometimes be difficult if there has been a placental fusion.
In a DCDA pregnancy, there are no vascular anastomoses between the twin placentae and with an equator marking a zone of disc fusion where there is a narrow band at the equator where chorionic vessels do not cross.
In a MCDA pregnancy, multiple vascular anastomoses are usually present making the junction less distinct with examples of arterial to venous, arterial to arterial, or venous to venous connections being evident .