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Twin-twin transfusion syndrome (TTTS) is a pathologic condition that occurs in identical twins (monozygotic) sharing one placenta (monochorionic). Almost all monochorionic twins exchange blood across the placenta, although usually, the exchange is balanced. In TTTS, the vascular connections in the placenta are abnormal and the blood passes unequally between the two fetuses, with one twin (the donor twin) pumping blood to the other (the recipient twin). Without intervention, the recipient twin presents with excessive amounts of amniotic fluid (polyhydramnios), becomes overhydrated, and may develop heart failure and die; the donor twin becomes severely dehydrated, presents with decreased amounts of amniotic fluid (oligohydramnios), and may die as well. Up to 90 percent of moderate to severe cases of TTTS without intervention may result in pregnancy loss.
TTTS appears in about 15 percent of monochorionic pregnancies, amounting to 2,000 sets of twins per year in the U.S.
TTTS is categorized into five stages based on the severity of the condition.
In stage 1, about 40 percent of the cases will remain stable or resolve spontaneously.
In stage 2 and above, the disease is progressive and will probably not resolve without treatment.
Abnormal blood flow patterns are seen in the umbilical artery or ductus venosus in either one or both twins.
Early detection of TTTS and close monitoring are critical to determining the appropriate management for each case. Treatment decisions are based on the stage of TTTS, maternal cervical length, the gestational age of the twins, and their overall health. Highly specialized medical care is essential for pregnancies affected by TTTS to optimize outcomes for both twins.
At Boston Children’s Hospital, our Fetal Care and Surgery Center specializes in the evaluation and treatment of TTTS and other complex fetal conditions.
At 19 weeks, Shannon discovered that her twins had twin-twin transfusion syndrome, a rare pregnancy complication.
In most pregnancies affected by TTTS, the pregnant person doesn’t show or feel symptoms, and the diagnosis relies solely on fetal ultrasound. However, extra amniotic fluid around the recipient twin may lead to discomfort, respiratory challenges, and premature contractions throughout the pregnancy.
Pathophysiology is based on unbalanced vascular connections in the shared placenta, leading to net transfer of fluid from one twin to the other. In addition, the fetus’ response to altered blood volume with vasoactive mediators may worsen the condition.
Monochorionic pregnancies require frequent and routine ultrasound evaluation to assess the health of the twins and rule out complications. Signs of TTTS may be noticed on a routine prenatal ultrasound, and other cases may be suspected when a uterus measures larger than it should for a particular week of pregnancy.
If a problem is suspected on your prenatal ultrasound, you may be referred to the Fetal Care and Surgery Center for more detailed imaging.
Other tests you may encounter include:
Magnetic resonance imaging (MRI) allows further investigation of other structural anomalies and placental pathologies.
There are several procedures used to treat TTTS. Which procedure or combination of procedures is right for you and your pregnancy depends on the severity of the condition.
Some cases of TTTS don’t progress past the earliest stage and thus require no intervention.
If TTTS progresses, your doctor may advise you to undergo one of the following procedures:
The outlook for TTTS depends on its severity and how far it has progressed. Moderate to severe TTTS pregnancies that receive fetoscopic laser photocoagulation of placental vascular connections present a significantly improved survival rate with up to 70 percent for both twins and 90 percent for at least one twin alive at birth. Survival varies depending on the disease stage, the size of the fetus, and ultrasound measurements.
With umbilical cord ligation, brain injury might be prevented in the surviving twin. Some survivors may do well and be completely healthy, while others may have injuries to their hearts and kidneys.
It is important to note that there is a risk of preterm delivery after any of these procedures.
