Twin-to-Twin Transfusion Syndrome


Background: Twin-to-twin transfusion syndrome (TTTS) is the result of an intrauterine blood transfusion from one twin to another twin. TTTS occurs in monochorionic, monozygotic twins. The donor twin often is smaller and anemic at birth. The recipient twin usually is larger and plethoric at birth.


Pathophysiology: TTTS is the result of transfusion of blood from one fetal twin to another twin. The blood transfusion from the donor twin to the recipient twin occurs through placental vascular anastomoses. The most common vascular anastomosis is a deep, artery-to-vein anastomosis through a shared placental cotyledon. TTTS is a specific complication of monochorionic twin pregnancies. A uniform nomenclature describing the spectrum of TTTS does not exist.

One classification scheme separates TTTS into severe, moderate, and mild. Severe TTTS presents early in the second trimester at 16-18 weeks' gestation. A difference exists in fetal size of twins who are more than 1.5 weeks gestation. Umbilical cord sizes differ between the twins. Hemoglobin concentrations usually are the same in both twins because the transfusion is a whole blood shift. Polyhydramnios develops in the sac of the recipient twin because of volume overload and increased fetal urine output. Oligohydramnios develops in the sac of the donor twin because of hypovolemia and decreased urine output. Severe oligohydramnios can result in the stuck twin phenomena in which the twin appears in a fixed position against the uterine wall.

Moderate TTTS develops later in the second trimester between 24-30 weeks gestation. Although a fetal size discrepancy exists at more than 1.5 weeks gestation, polyhydramnios and oligohydramnios do not develop. The donor twin becomes anemic, hypovolemic, and growth is retarded. The recipient twin becomes plethoric, hypervolemic, and macrosomic. Either twin can develop hydrops fetalis. The donor twin can become hydropic because of anemia and high-output heart failure. The recipient twin can become hydropic because of hypervolemia. The recipient twin also can develop hypertension, hypertrophic cardiomegaly, disseminated intravascular coagulation, and hyperbilirubinemia after birth.

Mild TTTS develops slowly in the third trimester. Polyhydramnios and oligohydramnios usually do not develop. Hemoglobin concentrations differ by more than 5 g/dL. Twin size differs by more than 20%. Acute TTTS can occur at birth during the time between clamping the umbilical cords of the first and second twin. During this interval, the second twin has sole connection to the placenta and may receive a significant transfusion of blood.



Mortality/Morbidity: Severe TTTS has a 60-100% mortality rate. Mild-to-moderate TTTS frequently is associated with premature delivery. Morbidity and mortality depend upon the gestational age of the twins at delivery. The more premature the twins are at birth, the higher the incidence of morbidity and mortality will be. Fetal demise of one twin is associated with neurologic sequelae in 25% of surviving twins.



Physical: TTTS can be suspected in pregnant women carrying twins if they develop a rapidly increasing fundal height. After birth, TTTS can be suspected if significant differences in the appearance of monozygotic twins exist.

Causes: TTTS occurs in monozygotic, monochorionic twin pregnancies when an anastomosis between placental vasculature exists.


Other Problems to be Considered:

Hydrops Fetalis
Polycythemia of the Newborn
Polyhydramnios and Oligohydramnios
Anemia of the newborn


Lab Studies:

Imaging Studies:


Medical Care: Detailed description of medical care of the pregnant woman carrying twins with TTTS is beyond the scope of this review. Medical care of twins after birth is directed toward problems related to prematurity, anemia, polycythemia, and hydrops fetalis.





Medical/Legal Pitfalls: