Identical twins share the same egg. When they start to grow in the womb, they usually share the same placenta. A single placenta, two umbilical cords, and two babies are the diagnostic signs of identical twins. Identical twins can be diagnosed by ultrasound imaging during the first trimester. If you get this surprising news, you need to know about twin-to-twin transfusion syndrome (TTTS). [1,2]
What Is It?
TTTS is a serious condition that only occurs in babies who share the same placenta. It may affect up to 15 percent of identical twin pregnancies, or about 6,000 babies each year. In very rare cases, it may occur in identical triplets. TTTS is a disease of the placenta, not the babies. The placenta is the blood supply for both twins. In TTTS, abnormal growth of blood vessels in the placenta sends more blood to one twin (the recipient twin) than to the other twin (the donor twin). Essentially the donor twin is giving a blood transfusion to the recipient twin inside the womb. 
Untreated TTTS can be dangerous for both babies. The earlier this condition develops, the more dangerous it is. The good news is that the condition can be treated.  Doctors do not know what causes TTTS. It does not run in families. It seems to be a chance occurrence. 
What Are the Signs and Symptoms in Mothers?
Identical twins share the same placenta but not the same amniotic fluid sac inside the womb. TTTS can develop quickly. The recipient twin gets more blood supply than needed and starts to produce lots of urine. Urine quickly fills the amniotic fluid sac. This may causes rapid growth of a mother’s belly early in pregnancy. This is usually the first sign of TTTS for a mom.  Mothers may also feel belly pain or tightness, have rapid weight gain, and have fluid retention. TTTS often causes premature labor. 
What Happens to the Twins?
If TTTS occurs very early in pregnancy, the donor twin may not survive. When TTTS occurs during the second trimester changes may occur in both twins. The donor twin may be smaller. This twin does not get enough blood and oxygen to grow normally. The recipient twin may grow quickly and start to retain fluid in the body and lungs. This can put pressure on the heart and cause heart failure.  After birth, both babies may have lingering digestive, respiratory, heart, or nervous system problems. 
Diagnosis of TTTS
Identical twins can be diagnosed at about 16 weeks of pregnancy. Signs of TTTS do not usually appear until the second trimester.  Women diagnosed with identical twin pregnancies should have an ultrasound done every week after week 16 to look for these signs. 
Diagnostic studies include ultrasound, fetal MRI, and fetal echocardiogram (ultrasound of the heart).  These studies may show: [2,3]
- Unequal size of the twins
- Unequal amniotic fluid sacs
- Abnormal blood flow through the placenta and umbilical cords
- Fluid buildup in the recipient twin
- Signs of heart failure in the recipient twin
Treatment of TTTS
Treatment depends on how severe the condition is and when it develops. In mild cases, the only treatment may be watchful waiting with frequent ultrasounds.  Early treatment may be removal of amniotic fluid from the recipient twin amniotic fluid sac. This is done though a long needle and is called therapeutic amniocentesis. This procedure may need to be repeated several times during pregnancy. [2-4]
For severe TTTS or TTTS that develops before 26 weeks, the best treatment is surgery to close the abnormal blood vessels in the placenta. This procedure is called fetoscopic laser coagulation. It can be done under local or general anesthesia. 
This surgery is done using a telescopic camera (fetoscope). The scope is inserted through a small incision in the belly. Once inside the womb, the surgeon identifies the blood vessels in the placenta that are diverting blood from the donor twin. These blood vessels are closed using a laser aimed through the scope to coagulate the abnormal blood vessels. 
Delivery and Prognosis for TTTS Twins
TTTS twins are more likely to be born prematurely. In some cases, the safest approach is to induce early labor once the twins have healthy lungs. Twins may spend some time in the neonatal intensive care unit. They may be followed closely to rule out any problems after birth. In most cases, both twins will do well and go on to live normal healthy lives. 
If you are diagnosed with an identical twin pregnancy, talk to your doctor about the possibility of TTTS. You may need a team of maternal and fetal care specialists. Make sure to show up for all your ultrasounds. Let your team know if you have symptoms of TTTS. With early diagnosis and specialized care, both you and your babies should do well.