May 15, 2026
Twin-twin transfusion syndrome (TTTS) is a rare yet serious complication of monochorionic twins requiring urgent fetal intervention. While about two-thirds of identical twins share a placenta, only about 10% to 15% of those twins develop TTTS.
Vascular communication is present in virtually all monochorionic twins. In the majority of these twins, the blood flow is balanced with no net transfusion of blood from one twin to the other. But in twins with TTTS, the blood flow through these anastomoses is unbalanced, and a net amount of blood is slowly transfused. Using ultrasound, physicians can identify this syndrome with a shared placenta and a significant amniotic fluid imbalance.
"If unaddressed, one fetus becomes hypovolemic and the other becomes hypervolemic," says Mauro H. Schenone, M.D., chair of Maternal and Fetal Medicine and a fetal surgeon at Mayo Clinic in Rochester, Minnesota.
Hypovolemia causes a decrease in the urine output leading to oligohydramnios or anhydramnios, while hypervolemia in the recipient twin causes an increase in the urine output that leads to polyhydramnios.
TTTS is staged by severity in five stages:
- Stage 1: Oligohydramnios and polyhydramnios are present.
- Stage 2: The donor twin's bladder is not visible on ultrasound.
- Stage 3: Progression to stage 3 involves further hemodynamic changes, noticeable with Doppler examination. At this stage, the risk of heart failure rises and intervention is often required. Without treatment at this stage, fetal loss risk reaches 75% to 100%.
- Stage 4: Fetal hydrops is present.
- Stage 5: Indicates the death of one or both twins.
Currently, there is no known genetic cause of twin-to-twin transfusion syndrome and research has yet to determine any proven risk factors for the condition. While little can be done in terms of prevention, there are clinical signs that may prompt evaluation. These signs include marked amniotic fluid discrepancy in a monochorionic diamniotic pregnancy, rapid maternal abdominal enlargement and symptoms related to polyhydramnios.
Screening typically begins at 16 weeks' gestation and is performed every two weeks thereafter. TTTS most commonly appears in the second trimester, but it can develop at any point during pregnancy. Regular biweekly ultrasound surveillance helps detect early fluid and Doppler changes so that timely management decisions can be made.
While in vitro fertilization (IVF) pregnancies have a slightly higher rate of identical twins, there have been no significant findings indicating an increase in TTTS rates among IVF pregnancies compared with spontaneous pregnancies.
Evolving treatments and techniques
Tools, techniques and outcomes have improved during the more than 30 years since the first documented laser-treated case in the U.S.
Fetoscopic laser photocoagulation (FLP) uses a fetoscope to locate and selectively ablate the placental vascular connections that cause the net transfusion. Mayo Clinic is one of the few medical centers offering treatment of TTTS worldwide.
"In our program, we have the new generation of fetoscopes that allows for better access and visualization of the placental anastomosis," Dr. Schenone says. "Our access to these tools really puts us in a very privileged position treating these cases."
The Solomon technique, also known as dichorionization, is another aspect of laser treatment's evolution. This approach is a refinement in which the surgeon creates a functional separation across the placental surface to reduce the risk of post‑laser twin anemia polycythemia sequence.
Though crucial for fetal survival, FLP is not without risk. The mothers may experience discomfort and pain, though generally transient and well tolerated. The procedure is by nature invasive, though the surgeon uses a minimally invasive technique.
FLP risks include:
- Fetal loss, especially in advanced cases.
- Premature rupture of membranes.
- Chorioamniotic separation.
- Premature delivery.
- Amniotic fluid leak into the maternal peritoneal cavity.
- Infection.
- Bleeding.
- Accidental injury to tissue or organs near the operative field.
FLP is the best treatment option for stages 2 through 4 and is approved by the Food and Drug Administration (FDA) for use during 16 0/7 to 25 6/7 weeks' gestation.
"Amnioreduction can relieve symptoms of polyhydramnios and is sometimes used to prolong pregnancy, but it does not correct the underlying vascular imbalance," Dr. Schenone says. "This treatment can be used in cases outside the FDA-approved gestational age for laser therapy."
Successful management requires a multidisciplinary team that includes fetal surgeons, maternal fetal medicine specialists, pediatric cardiology, neonatology, and experienced ultrasound sonographers and nursing staff.
"Preoperative evaluation is essential, and our highly skilled multidisciplinary team collaborates on the evaluation before we go into the OR," Dr. Schenone says. "Because TTTS is a rare occurrence, successful management requires a multidisciplinary team that consistently participates in FLP procedures and is familiar with the specialized equipment. It's imperative to have a team familiar with troubleshooting in this procedure and that can respond quickly and accurately."
For more information
Fetal and maternal care in Minnesota. Mayo Clinic.
Refer a patient to Mayo Clinic.