Fetal Intervention: Twin-to-Twin Transfusion Syndrome

Twin-to-twin transfusion syndrome (TTTS) is a condition that only occurs with monochorionic-diamniotic (MCDA) twins. With this syndrome, twins share a placenta and a network of blood vessels that supply oxygen and nutrients; however, the blood flow between the twins is unequal.

This condition can occur in up to 10-15 percent of MCDA twins, reports Translational Pediatrics. Diagnosis and treatment are extremely important as, without fetal intervention, this condition can be fatal for both twins.

Identifying Twin-to-Twin Transfusion Syndrome

The first step to evaluating a twin pregnancy for TTTS is to positively identify an MCDA pregnancy with a first-trimester ultrasound. Then, it must be determined if the twins are sharing one placenta. These assessments are best done with both a transabdominal and transvaginal ultrasound exam in order to accurately determine if the components of the syndrome are present.

In TTTS, the unequal sharing of blood between twins results in some noticeable structural changes in the pregnancy, which can be seen and documented with ultrasound. The unequal blood supply means that one twin (referred to as the donor) is actually pumping blood to the other twin (the recipient). This results in the recipient twin receiving too much blood and the donor twin receiving too little.

This causes a noticeable size disparity between the twins and a difference in the volume of amniotic fluid present around each fetus. This is because the donor twin — who is receiving too little blood — will produce less than normal amounts of urine, which leads to reduced amounts of amniotic fluid (oligohydramnios) within the amniotic sac and a small or absent bladder. In comparison, the recipient twin — who is receiving too much blood — will produce more than the normal amount of urine, resulting in a very large bladder and too much amniotic fluid (polyhydramnios), and can also lead to hydrops, which is a prenatal form of heart failure.

Carrying parents can experience abdominal pain from the excess amniotic fluid and are at risk of membrane rupture and premature labor if this condition is not treated.

Treating Twin-to-Twin Transfusion Syndrome

TTTS can lead to injury to the cardiovascular system of one or both of the twins. For this reason, a detailed ultrasound assessment of both twins, and especially their hearts, is necessary in any suspected cases of twin-to-twin transfusion syndrome.

The carrying parent of suspected TTTS twins will be referred to a specialist organization — such as the Center for Fetal Diagnosis and Treatment — for a comprehensive evaluation. This can include exams such as high-resolution obstetric ultrasound, fetal echocardiogram and genetic amniocentesis. These assessments are designed to ensure an accurate diagnosis, as TTTS can appear similar to other obstetric anomalies, such as intrauterine growth restriction.

Twin-to-twin transfusion syndrome treatment and fetal intervention will depend on the stage and severity of the disorder and can include any of the following:

- Fetoscopic selective laser ablation: A surgery on the placenta that obstructs the blood flow to the recipient twin through some placental blood vessels. This is the preferred method for treating TTTS, but it does depend on the stage of the condition and the location of the placenta.

- Amnioreduction: A temporary treatment that drains out some of the excess amniotic fluid from the recipient twin to relieve discomfort for the mother.

- Selective cord occlusion: A last-resort procedure that stops the blood flow from donor twin to recipient twin. It is used when TTTS is at a severe stage — to the point where one twin is not going to survive. This procedure can protect the other twin from death or neurological impairment.

Improving Survival Rates

Twin-to-twin transfusion syndrome is a rare and dangerous condition that requires fetal intervention to ensure the survival of the twins. Fortunately, the procedures available are often minimally invasive and, because of the use of ultrasound, can be targeted to specific areas of the placenta, protecting the fetus.

Moreover, a 2020 report in Ultrasound in Obstetrics and Gynecology offers hope that survival rates of TTTS are improving more and more every year — thanks in large part to advancements in imaging. Moving forward, it appears more patients and more little ones will receive the care they need with improved ultrasound technology.