Twin-to-Twin Transfusion Syndrome (TTTS) is a rare and severe complication that arises in pregnancies where identical twins share a single placenta (monochorionic twinning). This complication involves an imbalance in the blood supply between the developing fetuses, leading to serious health consequences for both. The syndrome is entirely a disease of the shared placenta and is not caused by genetic factors or anything the mother did during pregnancy. It progresses rapidly and requires specialized prenatal diagnosis and intervention to improve outcomes.
Understanding the Unequal Blood Flow
The mechanism of TTTS is rooted in the shared placenta, which contains vascular connections (anastomoses) between the twins’ circulations. In TTTS, the blood flow becomes chronically and directionally unbalanced. This unequal flow causes one twin, the “donor,” to continuously transfer blood volume to the other twin, the “recipient.”
The donor twin suffers from hypovolemia (low blood volume), which restricts growth and decreases blood flow to the kidneys. Reduced kidney function results in little or no urine output, the primary source of amniotic fluid late in gestation. This causes the donor twin to be surrounded by an abnormally small amount of amniotic fluid, called oligohydramnios.
Conversely, the recipient twin experiences hypervolemia (blood volume overload), straining the heart and circulatory system. The excess blood causes the recipient’s kidneys to produce a large amount of urine. This overproduction leads to an excessive accumulation of amniotic fluid, known as polyhydramnios.
Prevalence and Necessary Conditions
TTTS is considered rare because it requires a specific prerequisite: a monochorionic pregnancy, where identical (monozygotic) twins share a single placenta. Fraternal twins, which are dichorionic and have two separate placentas, are not at risk for TTTS.
Monozygotic twins occur in approximately three to five out of every 1,000 conceptions. About 75% of these are monochorionic and share a placenta. TTTS develops in roughly 10% to 15% of those monochorionic twin pregnancies.
The incidence of TTTS in the overall population is estimated to be about one in every 2,000 total pregnancies. This rarity requires the confluence of a monozygotic conception, shared placenta, and a significant flow imbalance. The condition most often develops during the second trimester, typically between 16 and 26 weeks gestation.
Diagnostic Tools and Severity Staging
Diagnosis of TTTS is established through regular ultrasound monitoring of monochorionic twin pregnancies. The first step confirms discordant amniotic fluid volumes in the separate sacs. Diagnosis requires the recipient twin’s sac to show polyhydramnios, defined as a single deepest vertical pocket of eight centimeters or more. Simultaneously, the donor twin’s sac must show oligohydramnios (a deepest vertical pocket measuring less than two centimeters).
Once fluid imbalances are confirmed, the condition is classified by severity using the Quintero Staging System, which guides treatment decisions.
Stage I
This is the least severe stage, characterized by fluid discordance, but the donor twin’s bladder is still visible on ultrasound.
Stage II
This stage occurs when the donor twin’s bladder is no longer visible, indicating severely reduced urine production.
Stage III
This stage is marked by abnormal blood flow patterns detected through Doppler ultrasound studies. These studies assess the velocity and direction of blood flow in vessels like the umbilical artery and ductus venosus.
Stage IV
This stage means the development of fetal hydrops, where the babies accumulate excess fluid in two or more body cavities, suggesting severe heart failure.
Stage V
This final classification indicates the death of one or both twins.
Interventional Treatment Options
The definitive treatment for Twin-to-Twin Transfusion Syndrome focuses on correcting the underlying cause in the placenta. Fetoscopic Laser Photocoagulation (FLPC) is the preferred and most effective intervention, targeting the unbalanced vascular connections.
The procedure involves inserting a tiny camera (fetoscope) into the recipient twin’s amniotic sac under local anesthesia and continuous ultrasound guidance. The surgeon uses the fetoscope to visualize the placenta and map the abnormal blood vessel connections. A laser fiber is then passed through the fetoscope to ablate, or seal off, these vessels, separating the twins’ circulations. This procedure directly addresses the root cause of the unequal blood transfer and is typically performed between 16 and 26 weeks of gestation.
For very early stages or as an initial measure, serial amnioreduction may be used. This procedure involves repeatedly draining the excess amniotic fluid from the recipient twin’s sac to relieve pressure and reduce the risk of preterm labor. However, laser surgery is the standard of care for advanced stages because it offers the only direct treatment for the placental vascular imbalance.