The vast majority of pregnancies unfold in a normal and healthy way, but occasionally there are problems that require highly specialized treatment. This happens more often to women with a multiple pregnancy (or gestation) such as twins or triplets.

Twin-to-twin transfusion syndrome (TTTS) is one such challenge but Yale Medicine has unique expertise in treating this problem. Mert Ozan Bahtiyar, MD, an obstetrician-gynecologist who specializes in high-risk pregnancies and is medical director of the Yale Medicine Fetal Care Center, is the only physician in the state who performs a procedure called laser fetoscopy to cure twin-to-twin transfusion syndrome.

TTTS is a rare, in utero condition that occurs only in monochorionic twins (identical twins who share the same placenta). Attached to the inside of the uterus during pregnancy and connected to the fetus by the umbilical cord, the placenta delivers nutrients and oxygen from the mother’s blood to the developing babies.

With twin-to-twin transfusion syndrome, which occurs in 10 to 15 percent of monochorionic twin pregnancies, there is an imbalance in blood flow due to placental vessel connections. One twin (called the “donor” twin) gives too much blood to the “recipient” twin.

The extra blood causes the recipient’s kidneys to produce more urine, which creates a large bladder. This can lead to too much amniotic fluid (polyhydramnios), prenatal heart failure or excessive swelling (hydrops).  Meanwhile, the donor twin doesn’t have any or enough amniotic fluid. Amniotic fluid is essential for a baby’s development, but too much or too little of it can cause serious pregnancy complications.

Twin-to-twin transfusion syndrome is a progressive condition, meaning it gets worse if not treated appropriately. It usually develops between weeks 16 and 26 of pregnancy and can be easily detected with ultrasound.

At Yale Medicine, our Fetal Care Center is experienced at diagnosing and treating twin-to-twin transfusion syndrome.

“Twin-to-twin transfusion syndrome can be a troubling diagnosis for a patient to receive, but the good news is that we offer a procedure that has very good results,” says Dr. Bahtiyar. If no action is taken, one or both twins will die in more than 80 percent of cases.

There is no known genetic or other cause of twin-to-twin transfusion syndrome.

“We don’t know why, in some pregnancies, this develops, but not in others,” Dr. Bahtiyar says.

“Twinning,” on its own, he points out, is poorly understood and has unknown causes. The risk of complications grows with each additional baby a mother carries during pregnancy. 

Twin-to-twin transfusion syndrome typically develops between weeks 16 and 26 of a woman’s pregnancy. But an ultrasound as early as 10 to 14 weeks can put doctors on alert that there is a twin gestation, and if they are sharing a placenta.

TTTS is diagnosed by measuring levels of amniotic fluid. If one twin has a high level of amniotic fluid in the amniotic sac and the other twin has a low level, it will be visible via ultrasound.

“Amniotic fluid on an ultrasound is a marker for how the baby is doing,” explains Katherine Kohari, MD, a high-risk pregnancy specialist. “On the ultrasound, we look at what is called the maximum vertical pocket, which should be between 2 and 8 centimeters for each fetus. Some of that is subjective, based on the gestational age—and amniotic fluid peaks usually in the middle of the third trimester, but it gives us a general sense.” 

Another tell-tale sigs of TTTS is drastically different weights and/or sizes of the twins. 

It is crucial to distinguish between TTTS and other rare disorders that can affect identical twin pregnancies, including selective intrauterine growth restriction (SIUGR) and twin anemia polycythemia sequence (TAPS).

“Most, but not all, of identical-twin pregnancies share a single placenta, which is divided into two parts. Each part supports one twin,” says Dr. Bahtiyar, explaining SIUGR. “The division of the placenta is random. It could be close to equal, or one part might be much larger than the second part. And the amount of the placenta is directly proportional to the amount of nutrition that goes to one fetus. If there is an abnormality in this division, one fetus might be very small compared to the other twin. In itself, this is a problem because the smaller baby will then be at risk.”

While SIUGR is more about a balance of resources, TTTS involves blood vessel connections. But since the symptoms (one twin being larger than the other) of both disorders could be similar and the treatments are not the same, it is important to know the difference. The Fetal Care Center, with its expertise and experience, can make such a distinction.

“The takeaway with telling the difference between SIUGR and TTTS from ultrasound is that with SIUGR, you see that the baby is really small, but you may not see any differences in the amniotic fluid,” Dr. Kohari says.

Another rare condition that can affect identical twins with a shared placenta is twin anemia polycythemia sequence (TAPS). “TAPS is a chronic form of blood transfusion problems due to minuscule vascular connections—artery to vein,” Dr. Bahtiyar says. “Twin-to-twin transfusion syndrome, on the other hand, is due to large caliber connections.”   

While all or some combination of these syndromes can occur at the same time, again, it is vital to properly diagnose what is going on inside the womb so that a treatment plan can be put in place. 

Twin-to-twin transfusion syndrome is organized into five stages:

• Stage I: There is an imbalance in amniotic fluid between the twins.
• Stage II: The bladder of the donor twin is “absent,” meaning it does not fill with urine and is not visible on an ultrasound exam.
• Stage III: The excessive blood flow starts to cause problems with the recipient twin’s circulation. This can be detected through abnormalities in blood-flow patterns, which can be seen on an ultrasound.
• Stage IV: The heart of the recipient twin begins to fail because of excessive fluid retention. This can be seen as fluid collections around the abdomen and the chest.
• Stage V: One or both twins die from severe TTTS.  

For stage I, the pregnancy will be monitored closely. “It can stop progressing beyond stage I,” Dr. Bahtiyar says. “We tell patients that, with stage I, a third get better, a third stay the same, and a third get worse. So, we don’t immediately recommend fetoscopic laser treatment.” 

If you are diagnosed with stage II or above, you have what’s called advanced stage disease, and your doctor may recommend fetoscopic laser treatment.  

Because the recipient twin receives too much blood, it can thicken. Thicker blood is harder to pump, so the baby is at risk for developing soft tissue swelling and heart failure, and may even die in utero. 

Meanwhile, the donor twin is at risk for organ failure, including the kidneys, because of inadequate blood flow. The death of one fetal twin leaves the other at high risk of severe brain damage or even death, because of the connected blood vessels across the shared placenta.

Laser fetoscopy (selective laser photocoagulation) is the preferred method of treatment for TTTS. Yale New Haven Hospital is one of only 30 to 40 hospitals in the United States offering the procedure. Laser fetoscopy can be safely done up until 26 weeks of gestation.

With laser fetoscopy, the physician makes a small incision in the mother’s abdomen and inserts a tiny instrument equipped with a laser, which burns the unequal blood vessel connections. Excessive amniotic fluid is then drained from the area around the recipient twin using a vacuum-assisted device.

The procedure, which only takes about 15 minutes, typically requires a one-night hospital stay and is significantly better in saving one or both babies compared to amnioreduction, which involves draining large volumes of amniotic fluid in hopes of slowing down TTTS. 

The biggest risk with laser fetoscopy, Dr. Bahtiyar explains, is rupturing the amniotic sac, which could lead to premature labor. 

Twin-to-twin transfusion syndrome is one of many complicated conditions that our Fetal Care Center specialists commonly treat. We receive referrals from around the Northeast for TTTS and are the only center in the state offering laser fetoscopy treatment.  

In addition to expert diagnosis and treatment, our doctors and other health care providers are highly attuned to a family’s emotional needs. Our care coordinator keeps parents up-to-date with the latest test results and helps set up any appointments needed with pediatric subspecialists. We also offer a support group and access to a social worker, when needed.

“There is no other institution in Connecticut that has a Fetal Care Center,” Dr. Bahtiyar says.