Twin–twin transfusion syndrome (TTTS) is a complication of monochorionic twinning, resulting from unbalanced shunting of blood from one twin (the donor) to the other (the recipient) through arteriovenous anastomoses in their common placenta. The donor twin becomes anemic and growth restricted, while the recipient becomes polycythemic. There are also discrepant amniotic fluid volumes: oligohydramnios in the donor twin’s sac and polyhydramnios in the recipient’s sac. In severe cases, the recipient may develop hydrops.

TTTS occurs in approximately 10% of monochorionic twins. Complications may arise in the donor as a result of anemia and growth restriction and in the recipient due to high-output congestive heart failure. The mortality rate is high for both the donor and recipient twins. Treatment options include therapeutic amniocentesis from the sac of the recipient twin and endoscopically guided laser coagulation of communicating placental vessels. Use of the former treatment in mild to moderate cases of twin–twin transfusion syndrome and the latter in severe cases appears to increase the likelihood of the pregnancy resulting in at least one surviving fetus.

Twin anemia–polycythemia sequence (TAPS) is a variant of TTTS in which there is chronic low-volume transfer of blood from one twin to its monochorionic co-twin through small arteriovenous anastomoses in their common placenta. The amniotic fluid and size differences seen with TTTS are not present, so that the diagnosis is often not made until after birth. At that point, TAPS can be suspected when one twin is pale and the other plethoric, and can then be confirmed by blood tests showing one twin to be anemic and the other polycythemic.

The sonographic diagnosis of twin–twin transfusion syndrome is typically made when three findings are present:

In approximately 10% of cases of twin–twin transfusion syndrome, the recipient twin is hydropic (Figure 24.1.3).

The diagnosis of TAPS is made on the basis of middle cerebral artery Doppler. The characteristic finding is high velocity of greater than 1.5 multiples of the median in one twin (the anemic one) and low velocity (usually defined as below 0.8 multiples of the median) in the other.

Acardiac twinning is a rare complication of monochorionic gestations. It occurs as a result of artery-to-artery and vein-to-vein anastomoses across the common placenta, which disrupt the hemodynamic balance between the twins, such that the cardiovascular system of one twin takes over the cardiovascular system of its co-twin. The twin whose cardiovascular system takes over is termed the pump twin, and the other twin, whose heart usually fails to develop, is termed the acardiac twin. The pump twin’s heart propels blood through its umbilical arteries into the placenta and forces blood across the artery-to-artery anastomoses. This arterial blood then travels in a reverse direction, from the placenta through the acardiac twin’s
umbilical artery to the acardiac twin. The acardiac twin receives its oxygen and nutrition from the incoming arterial blood that originated in the pump twin. Blood travels passively through the acardiac twin and returns through its umbilical vein to the placenta, where it crosses the vein-to-vein anastomoses back to the pump twin. Because blood in the acardiac twin is in the reversed direction of normal, entering via its umbilical arteries and exiting via its umbilical vein, the abnormality is termed the twin reversed arterial perfusion (TRAP) sequence.

As a result of the altered hemodynamics, the heart in the acardiac twin is absent or rudimentary. In addition, this anomalous twin often has an abnormal or absent head, small, poorly formed upper extremities, and a two-vessel cord. The lower half of its body tends to be more normally formed, often with normal-appearing spine, kidneys, bladder, and lower extremities. The entire acardiac twin is typically surrounded by massive skin edema, particularly in the upper portion of the body. The acardiac twin continues to grow during gestation.

The prognosis for the pump twin is related to whether this twin develops congestive heart failure in utero and to the size of the acardiac twin. Large acardiac twins may receive a large amount of blood, forcing the pump twin to increase cardiac output significantly. This can lead to hydrops or demise of the pump twin. Overall, the survival rate of pump twins is approximately 50%. The prognosis can be improved if the umbilical cord of the acardiac twin is ligated or occluded with laser before the pump twin develops high-output heart failure.

The amnionicity of an acardiac twin gestation may be monoamniotic or diamniotic.