The twin-to-twin transfusion syndrome occurs in monochorionic, same gender twin pregnancies when blood is transferred from one twin (the donor) to the second twin (the recipient) through deep arteriovenous anastomoses in shared lobules. The sharing of blood may be unidirectional or bidirectional, depending on the nature of the anastomoses. If the transfer of blood from the donor to the recipient is excessive, then differences between the two infants can be seen on ultrasonography and at birth.

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