Twin-to-Twin Transfusion Syndrome (TTTS) is a serious complication that affects identical twin pregnancies where the babies share a single placenta, a condition known as monochorionic gestation. This syndrome results from an imbalanced sharing of blood flow between the fetuses through the connecting blood vessels on the placental surface. TTTS is estimated to occur in 10 to 15 percent of monochorionic twin pregnancies and, if left untreated, carries a high rate of mortality and morbidity for both babies. Early detection through regular prenatal surveillance is necessary for timely intervention, which significantly improves the outcomes for both twins.
How Twin-to-Twin Transfusion Syndrome Develops
TTTS develops when the transfer of blood across the vascular connections (anastomoses) in the shared placenta is significantly unbalanced and predominantly unidirectional. This imbalance is primarily driven by deep arteriovenous connections where blood flows from an artery of one twin into a vein of the other.
This continuous, uneven transfer of blood results in two distinct physiological states, defining the “donor” and “recipient” twins. The donor twin consistently loses blood volume, leading to low blood volume and anemia. This reduced blood volume causes the donor twin to produce very little urine, resulting in a low level of amniotic fluid.
Conversely, the recipient twin receives an excessive amount of blood volume, which can lead to an abnormally high concentration of red blood cells (polycythemia). This volume overload forces the recipient twin’s kidneys to produce a large amount of urine, leading to an excessive accumulation of amniotic fluid. The recipient twin’s heart must work harder to pump the extra fluid, which can ultimately lead to heart failure and generalized swelling (hydrops).
Diagnosis and Severity Staging
Diagnosis of TTTS relies on specific findings observed during a prenatal ultrasound examination. The primary diagnostic criteria include an amniotic fluid volume discrepancy: the recipient twin must have a maximum vertical pocket (MVP) greater than 8 centimeters, while the donor twin has an MVP of less than 2 centimeters. Other signs include a large bladder in the recipient twin and a small or non-visible bladder in the donor twin.
Once diagnosed, the severity of the syndrome is categorized using the Quintero Staging System, which guides management decisions. This system is based on five progressive stages. Stage I is the least severe, defined by the amniotic fluid discrepancy while the donor twin’s bladder is still visible and blood flow studies (Doppler) are normal in both twins.
Progression to Stage II occurs when the donor twin’s bladder is no longer visible on ultrasound, indicating severely reduced urine output, but Doppler studies remain normal. Stage III signifies abnormal blood flow patterns in either twin, detected by Doppler ultrasound, such as absent or reversed flow in the umbilical artery, ductus venosus, or umbilical vein.
Stage IV is reached when either twin develops hydrops fetalis, which is the accumulation of fluid in at least two different body areas, typically a sign of impending heart failure. The most severe classification is Stage V, which is diagnosed when one or both fetuses have died in utero.
Medical Interventions for TTTS
Treatment for TTTS is determined by the Quintero stage. For the mildest cases, particularly Stage I, expectant management is often chosen. Intervention is usually initiated for Stage II and higher, or for Stage I cases that cause maternal discomfort or show signs of progression.
One non-surgical intervention is serial amnioreduction, which involves using a needle to drain the excess amniotic fluid from the recipient twin’s sac. This temporarily relieves maternal discomfort and reduces the risk of preterm labor. However, amnioreduction does not address the underlying issue of the unbalanced blood flow across the placenta.
Fetoscopic Laser Ablation (FLA) of the placental anastomoses is the definitive treatment for TTTS. This minimally invasive surgery involves inserting a tiny camera (fetoscope) into the amniotic sac to visualize the connecting blood vessels on the placenta’s surface. A laser fiber is then used to coagulate and seal these communicating vessels, effectively separating the twins’ circulations.
FLA is considered the gold standard because it treats the root cause of the syndrome. The procedure is typically performed between 16 and 26 weeks of gestation. Although highly effective, FLA carries risks, including preterm rupture of membranes and preterm labor, with a cumulative risk for complications around 10 to 15 percent.
Post-Birth Outcomes and Long-Term Monitoring
Following successful intervention or delivery, the most common complication for TTTS survivors is prematurity. Premature birth itself is a major factor contributing to later health issues.
Survivors are at an elevated risk for neurodevelopmental impairment (NDI), which can range from mild learning difficulties to more severe conditions like cerebral palsy. Recipient twins, due to the volume overload, also have a higher incidence of congenital heart disease and cardiac dysfunction that may require ongoing monitoring.
Long-term follow-up care is necessary for TTTS survivors. This monitoring often includes specialized developmental assessments and cranial ultrasounds to check for any signs of brain injury that may have occurred in utero or around the time of birth. Continued specialist care helps to identify and manage any potential neurodevelopmental or cardiac challenges early.