In single pregnancies, one placenta supports a single fetus. While fraternal twins — twins that come from two eggs and two sperm — are always surrounded by their own sacs and have their own individual placentas, 70 percent of identical twins may share a single placenta. These are called monochorionic twins.
Because there is no barrier separating the two fetuses in monochorionic twin pregnancies, there are almost always blood vessel connections in the placenta shared by the two fetuses. In about 10 to 15 percent of monochorionic twins, abnormal or imbalanced blood vessel connections in the shared placenta can cause an imbalance in the circulations of the fetuses. There may be significant transfer of blood from one twin — the so-called "donor twin" — to the other twin, called the "recipient." This results in twin-to-twin transfusion syndrome, or TTTS. TTTS is a serious, progressive disorder.
In TTTS, an artery branches off from the donor twin's umbilical cord, entering the placenta to obtain oxygen and nutrients from the mother's circulation. Unfortunately, the corresponding vein that would normally bring the now nutrient-rich blood back to that same fetus is instead directed toward the other twin via this abnormal arterio-venous connection. If there are no connections flowing in the opposite direction, one twin receives too much blood, and the other too little.
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.
In TTTS, the recipient twin's blood can become thick and difficult to pump around the body. This may lead to heart failure, generalized soft tissue swelling, and in some cases, fetal death. The donor twin is at risk for failure of the kidneys and other organs because of inadequate blood flow.
Because of the blood vessels that connect the circulations of the two fetuses across the shared placenta, if one twin dies, the other twin faces significant risk of death or damage to vital organs. If one twin dies, the surviving twin has up to a 40 percent risk of some form of brain injury. Unfortunately, without treatment, about 70 to 80 percent of twins with TTTS will die. Survivors may have injuries to their brains, hearts or kidneys.
With fetoscopic laser intervention, a treatment performed during pregnancy, the outcome is more hopeful. At UCSF's Fetal Treatment Center — one of the first centers in the world to use fetoscopic laser intervention for TTTS — survival rates for at least one twin are greater than 85 percent, and for both twins are approximately 60 percent.
Because TTTS is a progressive disorder, early treatment may prevent complications such as preterm labor and premature rupture of membranes due to excessive amniotic fluid. Treatment depends on the severity of the TTTS and the stage of the pregnancy.
All patients with stage II, III or IV TTTS, as well as some patients with stage I TTTS, should learn about and consider fetal intervention. In most cases, the optimal treatment is fetoscopic laser intervention. The UCSF Fetal Treatment Center was one of the first in the world to perform fetoscopic laser intervention to treat TTTS. At UCSF, survival rates for at least one twin are greater than 85 percent, and about 60 percent for both twins with this procedure.
Reviewed by health care specialists at UCSF Benioff Children's Hospital.
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