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.


The donor infant is typically the infant with the lower birth weight, lower hemoglobin level, and oligohydamnios (due to reduced urine production). The recipient infants typically has the higher birth weight, higher hemoglobin and polyhydramnios (due to increased urine output).

(1) If the donor twin becomes sufficiently anemic, then heart failure and fetal hydrops may occur.

(2) The recipient twin may become polycythemic.


Ultrasonographic criteria for twin transfusion syndrome:

(1) striking discrepancy in size of twins

(2) polyhydramnios surrounding the larger twin

(3) oligohydramnios surrounding the smaller twin ("stuck twin" sign)

(4) monochorionic placenta

(5) same sex of twins


The presence of adult RBCs in the circulation of the recipient twin after transfusion of adult blood into the donor twin by cordocentesis is considered the definitive criterion by some investigators.

(1) Fetal cells are rich in i-antigen and show little I-antigen, so that they react strongly with anti-i but only weakly if t all with anti-I.

(2) Adult cells are rich in I-antigen and show little i antigen, so that they react strongly with anti-I but only weakly if at all with anti-i.


Neonatal criteria for diagnosis of the twin transfusion syndrome:

(1) demonstration of vascular anastomoses between two placental territories using injection studies (where a distinctive fluid or air is injected into a blood vessel on one side which appears to be connect with a blood vessel on the opposite territory, and showing flow into the remote vessel)

(2) intertwin hemoglobin difference > 5 g/dL

(3) intertwin birth weight difference > 20%


Limitations on neonatal criteria:

• Compensatory reticulocytosis in the donor twin may reduce the discrepancy between the hemoglobin concentrations.

• Acute intrapartum twin transfusion is not associated with a discrepancy in birth weight.


Additional findings:

(1) One or both of the twins may be stillborn.

(2) Examination of the placenta often shows pallor of the villous parenchyma on the donor side with collapsed capillaries in the villi, while the recipient's side shows congested villous parenchyma and dilated capillaries.


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