Twin-To-Twin Transfusion Syndrome

When a fetus doesn't have enough blood and oxygen, it tries to use what it has most efficiently. Blood is shunted preferentially to the most important organs, the brain and the heart, and away from less vital organs like the kidneys.

This causes the kidneys to partially shut down, and the fetus makes less urine. Because amniotic fluid is mostly comprised of fetal urine, the reduced urine output causes low amniotic fluid levels, called oligohydramnios. As the kidneys make less and less urine and the oligohydramnios worsens, the fetal bladder may empty and will no longer be visible by ultrasound, since it is not being filled with urine.

Meanwhile, the recipient becomes overloaded with fluid as a result of the ongoing blood transfusion from the donor twin, and responds by producing large amounts of urine. This leads to very large amounts of amniotic fluid in the recipient's sac, called polyhydramnios.

An ultrasound showing this combination of oligohydramnios and polyhydramnios in a monochorionic twin pair indicates the diagnosis of TTTS. True TTTS is diagnosed when ultrasound examination shows that the deepest pocket of amniotic fluid in one twin's sac measures less than 2 centimeters, while the deepest pocket of amniotic fluid measures greater than 8 centimeters in the other twin's sac.

Although TTTS is diagnosed based on the amniotic fluid levels in each sac, the twins may also differ significantly in weight and size. Some of the size differences may be due to the TTTS process. However, much of it is due to the different portion of the placenta devoted to each twin, or unequal placental sharing.

Most monochorionic twins that develop TTTS also have some unequal placental sharing, with a smaller portion of the placenta assigned to the donor twin. Many twins that only have unequal placental sharing, but are not transfusing one another, may be incorrectly diagnosed as having TTTS. Differences may be subtle, but outcomes are dependent on accurate diagnosis, and the treatment and management are different for each condition.

Careful ultrasound is crucial for correctly detecting and diagnosing TTTS. At the UCSF Fetal Treatment Center, ultrasounds are performed by specialists widely renowned for their diagnostic skills and expertise in this field, who have written textbooks and many scientific articles about fetal conditions such as TTTS and unequal placental sharing.

Evaluating the Severity of TTTS

The severity of TTTS is partially based on when the condition becomes evident. The earlier it presents, the more serious the problem. In addition, the degree of fluid imbalance between the twins is important in staging TTTS.

A bladder that remains empty in the donor twin is a concerning sign, indicating a more advanced stage of TTTS. The situation worsens further when, in addition to the abnormal discrepancy in fluid volumes, ultrasound shows abnormal blood flow patterns in the umbilical cord vessels of either one or both of the twins. Finally, evidence of heart failure and tissue swelling, called hydrops, in either twin — usually the recipient — indicates a very serious, advanced stage.

Many patients in whom TTTS is suspected may, on further investigation, be found to have twins with discrepant fluid volumes that do not meet the definition for stage I TTTS. Still, all patients carrying monochorionic twins with significantly unequal amniotic fluid volumes or fetal weights should be evaluated and followed very carefully for changes, as true TTTS can develop and worsen rapidly.

Fetal Echocardiography

To further evaluate the severity of TTTS, UCSF often performs fetal echocardiography. Fetal echocardiograms are specialized ultrasound studies of the fetal heart, performed by pediatric cardiologists with special expertise in this area.

Early signs of heart failure are usually seen first in the recipient twin, as its heart must work hard to pump the extra blood. These exams may reveal increased size of some of the heart chambers, and changes in flow across the heart valves. If the stress and overload on the recipient continues untreated, progressive changes may include decreased function of the heart chambers and possibly narrowing of one of the heart valves, called pulmonary stenosis.

Umbilical Artery Blood Flow

Finally, using information from both the echocardiogram and ultrasound exam, we look for blood flow patterns in the umbilical artery and vein and other major fetal blood vessels.

Blood in the umbilical artery normally flows away from the fetus and toward the placenta to obtain fresh oxygen and nutrients from the mother's circulation. If a placental condition worsens, it becomes harder for the blood to flow toward and within the placenta. With each heartbeat, the fetus pushes blood toward the placenta (the systole phase) through the umbilical artery, and normally, that beat is strong enough for blood to keep flowing forward, toward the placenta, even as the heart re-fills for its next beat (the diastole phase).

In some cases, as TTTS progresses, forward flow in the umbilical artery of the donor may diminish between heartbeats. If the condition worsens, there may be no flow or even reversal of flow direction during the re-filling (diastole phase) of the fetal heart.

All the echocardiogram and ultrasound exam findings are considered in determining the severity of TTTS for each individual pregnancy.

Reviewed by health care specialists at UCSF Benioff Children's Hospital.

Related Information

UCSF Clinics & Centers

Fetal Treatment Center
1855 Fourth St., Second Floor, Room A-2432
San Francisco, CA 94158
Phone: (800) 793-3887
Fax: (415) 502-0660
Appointment information

Intensive Care Nursery
1975 Fourth St., Third Floor
San Francisco, CA 94158
Phone: (415) 353-1565
Fax: (415) 353-1202

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