Twin-to-twin transfusion syndrome (TTTS) is a serious pregnancy complication where identical twins sharing a single placenta exchange blood unevenly, causing one twin to get too much blood and the other too little. It affects roughly 10 to 15 percent of monochorionic diamniotic twin pregnancies, which are pregnancies where identical twins share a placenta but have separate amniotic sacs. Without treatment, TTTS can be fatal for one or both twins, but modern interventions give both babies a strong chance of survival.
How TTTS Develops
Identical twins who share a placenta almost always have blood vessels on the placental surface that connect their two circulatory systems. About 95 percent of these pregnancies have such connections. In most cases, blood flows back and forth between the twins in roughly equal amounts, and the pregnancy proceeds normally.
TTTS develops when this exchange becomes unbalanced. Blood flows preferentially from one twin (the “donor”) to the other (the “recipient”), creating a cascade of problems for both. But the condition is more complex than a simple transfer of red blood cells. Both twins typically have similar hemoglobin levels, which means TTTS is not primarily about one baby becoming anemic while the other becomes blood-rich. Instead, the core problem is fluid imbalance. Hormones that regulate blood pressure and fluid volume pass between the twins through the shared vessels, disrupting each baby’s ability to manage its own circulation. The donor becomes volume-depleted while the recipient becomes volume-overloaded.
What Happens to Each Twin
The two babies experience opposite but equally dangerous problems.
The donor twin loses fluid volume and produces less and less urine. As urine output drops, the amniotic fluid around this baby shrinks dramatically. In advanced cases, the amniotic sac contracts so tightly around the donor that the baby appears “stuck” against the uterine wall, barely able to move. The donor’s bladder may become completely empty and invisible on ultrasound. Without adequate blood flow, the donor’s kidneys and other organs are at risk of failure. These babies are often noticeably smaller than their twin.
The recipient twin faces the opposite problem: too much fluid. This baby’s body is flooded with extra volume, and its blood becomes thick and difficult to pump. The heart has to work harder and harder to circulate this heavy blood. Over time, the recipient can develop heart failure and generalized swelling throughout the body, a condition called hydrops. Meanwhile, the amniotic fluid around the recipient balloons to excessive levels, contributing to the mother’s rapid uterine growth.
Signs and Diagnosis
Most mothers with TTTS don’t feel symptoms, and the condition is caught during routine ultrasound. When symptoms do occur, they tend to reflect the excessive amniotic fluid building up around the recipient twin: rapid growth of the uterus (measuring larger than expected for the gestational age), a feeling of abdominal tightness or pain, and sudden weight gain.
On ultrasound, the hallmark finding is a stark difference in amniotic fluid between the two sacs. The recipient’s deepest fluid pocket measures more than 8 centimeters, while the donor’s deepest pocket measures less than 2 centimeters. Doctors also look for differences in estimated fetal weight between the twins and check whether the donor’s bladder is visible. An empty, invisible bladder in the donor is a concerning sign that signals a more advanced stage.
Stages of Severity
TTTS is classified into stages based on how far the imbalance has progressed. In the earliest stage, the fluid discrepancy is present but both twins still show bladder filling and relatively normal blood flow. As it advances, the donor’s bladder becomes invisible, blood flow patterns in one or both twins become abnormal on Doppler ultrasound, and one or both twins may develop signs of heart failure or hydrops. The most severe stage involves the death of one or both twins. TTTS can progress quickly or remain stable at an early stage for weeks, which is why frequent monitoring is essential once it’s identified.
Treatment With Laser Surgery
The most effective treatment is fetoscopic laser surgery. A thin scope is inserted through the mother’s abdomen into the uterus, and the surgeon uses a laser to seal off the shared blood vessels on the placental surface. This permanently separates the two circulatory systems, stopping the unequal exchange at its source.
Outcomes from this procedure are encouraging. Data from one high-volume center covering more than 700 patients showed that both babies survived in 74 percent of cases, with at least one baby surviving in an additional 15 percent. That means roughly 89 percent of families left with at least one healthy baby.
An older alternative, amnioreduction, involves draining excess amniotic fluid from the recipient’s sac to relieve pressure. This treats the symptoms but doesn’t address the underlying blood-sharing problem. It’s sometimes described as a bandage rather than a fix. Amnioreduction is still used in situations where laser surgery isn’t an option, such as when the pregnancy falls outside the gestational age window where laser treatment is approved.
Long-Term Outlook for Survivors
The majority of TTTS survivors who undergo laser surgery develop normally. A systematic review of follow-up studies from the past decade found that about 9.7 percent of survivors experienced severe neurodevelopmental impairment, and roughly 5.1 percent were diagnosed with cerebral palsy. These rates have remained stable over time as surgical techniques have matured. The remaining 90 percent of survivors showed typical development, though many centers recommend developmental follow-up through early childhood to catch any subtle delays.
Both the donor and recipient can face challenges after birth. Former recipients may need monitoring for heart function, since their hearts worked harder in utero. Former donors may need kidney function assessments. In most cases, these concerns resolve in infancy, but they underscore why specialized postnatal care matters for TTTS pregnancies.