Twin-to-Twin Transfusion Syndrome

Updated: Aug 03, 2018
  • Author: Terence Zach, MD; Chief Editor: Ronald M Ramus, MD  more...
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Twin-to-twin transfusion syndrome (TTTS) is the result of an intrauterine blood transfusion from one twin (donor) to another twin (recipient). TTTS only occurs in monozygotic (identical) twins with a monochorionic placenta. The donor twin is often smaller with a birth weight 20% less than the recipient's birth weight. The donor twin is often anemic and the recipient twin is often plethoric with hemoglobin differences greater than 5 g/dL.



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. [1]

TTTS is a specific complication of monozygotic twins with monochorionic placentation. Monozygotic twins that have a dichorionic placentation are not at risk for TTTS. Monozygotic twins with monochorionic, diamniotic placentation or monochorionic, monoamniotic placentation are at risk for TTTS (see images below). [2]

Monozygotic twins with monochorionic, diamniotic p Monozygotic twins with monochorionic, diamniotic placentation.
Monozygotic twins with monochorionic, monoamniotic Monozygotic twins with monochorionic, monoamniotic placentation.

The clinical features of TTTS are the result of hypoperfusion of the donor twin and hyperperfusion of the recipient twin.

The donor twin becomes hypovolemic and oliguric or anuric. Oligohydramnios develops in the amniotic sac of the donor twin. Profound oligohydramnios can result in the stuck twin phenomenon in which the twin appears in a fixed position against the uterine wall. Ultrasonography typically fails to visualize the fetal bladder because of absent urine.

The recipient twin becomes hypervolemic and polyuric. Polyhydramnios develops in the amniotic sac of the recipient twin.

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 can also develop hypertension, hypertrophic cardiomegaly, disseminated intravascular coagulation, and hyperbilirubinemia after birth.




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Monozygotic twins occur in 3-5 per 1000 pregnancies. Monozygotic twins can be monochorionic or dichorionic. Approximately 75% of monozygotic twins are monochorionic. Only monochorionic twins are at risk for TTTS. TTTS occurs in 5-38% of monochorionic twins.

Thus, TTTS only occurs in same sex, monozygotic twins with monochorionic placentation.



Outcome is dependent upon gestational age at birth and whether intrauterine fetal brain ischemia occurred. The lower the gestational age at birth the greater the risk for long-standing neurologic or pulmonary sequelae. Catch-up growth occurs postnatally in most of the smaller donor twins.


Severe TTTS has a 60-100% fetal or neonatal mortality rate. Mild-to-moderate TTTS is frequently associated with premature delivery. Fetal demise of one twin is associated with neurologic sequelae in 25% of surviving twins. Fetal blood pressure instability can lead to brain ischemia in either the donor or recipient twin. Ischemia of the fetal brain can result in periventricular leukomalacia, porencephaly, microcephaly and cerebral palsy. The more premature the twins are at birth, the higher the incidence of postnatal morbidity and mortality.

In a review of 135 monochorionic twin pregnancies with single intrauterine death (sIUD), whether spontaneous or procedure related, O'Donoghue et al found that death of the co-twin followed in 22.9% of cases. In the pregnancies that continued after sIUD, the frequency of antenatally acquired brain injury in the co-twin was significantly lower after procedure-related than spontaneous sIUD: 2.6% versus 22.2% (P = 0.003). The investigators conclude that the risk of brain injury is reduced but not negated by procedures that restrict inter-twin transfusion. [3]


Neurologic sequelae

Intrauterine demise of one twin can result in neurologic sequelae in the surviving twin. Acute exsanguination of the surviving twin into the relaxed circulation of the deceased twin can result in intrauterine CNS ischemia.