Most pregnancies result in one baby. In about one in 80 pregnancies, twins are conceived.
This can occur in one of two ways:
The more common way (which accounts for two-thirds of cases)
is for the two different sperm to fertilize two different eggs, resulting in what
is called a dizygotic (DZ) twin gestation. These twins are often called fraternal
twins. In this type of twinning each twin has its own sac of amniotic fluid and
its own placenta (afterbirth). Dizygotic twins have two sets of membranes surrounding
their amniotic fluid sacs (one inner amnion layer and one outer chorion layer),
and therefore they are known as diamniotic, dichorionic.
In about one-third of twin pregnancies, one sperm fertilizes
one egg, but this splits into two embryos resulting in what is known as monozygotic
(MZ) twins. These twins are often referred to as identical twins since they have
the same genetic material. Approximately one-third of MZ twins look just like fraternal
twins on prenatal ultrasound since there are two separate amniotic sacs and two
separate placentas. However, in two-thirds of identical twins, each twin has its
own amniotic sac but shares a common placenta. This type of MZ twinning is called
monochorionic, diamniotic since there is an inner layer surrounding the amniotic
sac of each twin, but there is only one common outer layer (chorion) surrounding
both of the sacs. This type of twinning occurs in approximately one in 360 pregnancies.
Monochorionic twins are at higher risk for complications since they share a common
Less than 1 percent of identical twins (about one in 2,400
pregnancies) will have one amniotic sac and one placenta for both twins. This type
of twinning is referred to as monchorionic, monoamniotic
twinning. These twins are at very high risk for loss of the pregnancy due to entangled
This condition occurs only in those identical twins that
are monochorionic, diamniotic (one-third of all identical or monozygotic twins).
In almost all of these pregnancies, the single placenta contains blood vessel connections
between the twins. For reasons that are not clear, in 15 percent to 20 percent of
monochorionic, diamniotic twins, the blood flow through these blood vessel connections
becomes unbalanced, resulting in a condition known as twin-twin transfusion syndrome
(TTTS). This is not an inherited or genetic condition. It is not caused by something
that a mother or father has done or not done.
In TTTS, the smaller twin (often called the donor twin) does not get enough blood
while the larger twin (often called the recipient twin) becomes overloaded with
too much blood.
In an attempt to reduce its blood volume, the recipient twin will increase the urine
it makes. This will eventually result in the twin having a very large bladder on
ultrasound, as well as too much amniotic fluid around this twin. This is known as
At the same time, the donor twin will produce less than
the usual amount of urine. The amniotic fluid around the donor twin will become
very low or absent. This is known as oligohydramnios.
As the disease progresses, the donor will produce so little
urine that its bladder may not be seen on ultrasound. The twin will become wrapped
by its amniotic membrane (known as a “stuck” twin). Often the polyhydramnios of
the recipient twin is the first thing noticed by the patient due a sudden increase
in the size of the uterus. Clothes may become tight fitting over a short period
of time. At other times the differences in the amniotic fluid volumes between the
twins is only noted at the time of a routine ultrasound.
How is Twin-Twin Transfusion Syndrome diagnosed?
The diagnosis of TTTS is made with an ultrasound evaluation that shows a twin pregnancy
with one placenta, twins of the same sex in separate amniotic sacs, and polyhydramnios
in the recipient’s sac and oligohydramnios in the donor’s sac. Amniotic fluid volume
is measured with ultrasound by determining the deepest pocket measurement from the
patient’s skin to her back. This measurement is known as the maximum vertical pocket
What are the Twin-Twin Transfusion Syndrome five stages of classification?
Quintero1 has proposed five stages
of TTTS based on ultrasound findings:
Stage I: This is the initial way that TTTS is seen on ultrasound. In stage
I, there is oligohydramnios in the donor’s sac with an MVP of two centimeters or
less (three-quarters of an inch) and polyhydramnios in the recipient’s sac with
a maximum vertical pocket of fluid of eight centimeters or more (just over three
inches). The bladder of the donor baby is still seen.
Stage II: As defined above, there is polyhydramnios and oligohydramnios,
but the bladder is no longer seen in the donor twin during the ultrasound evaluation.
Stage III: Blood flow in the fetus can be measured with a special type of
ultrasound called Doppler. In addition to the findings of Stages I and II, careful
study of the blood flow in the umbilical cord and fetal ductus venosus (the large
blood vessel in the fetus that returns blood to the heart from the placenta) reveals
abnormal patterns in Stage III. These patterns can occur in either or both fetuses.
In the umbilical cord, the diastolic flow can be either absent or reversed in the
umbilical artery. This pattern is usually seen in the donor twin. In the ductus
venosus, the diastolic flow can either be absent or reversed. This pattern is usually
see in the recipient twin due to early heart failure. The recipient twin can also
exhibit leakage across the main valve on the right side of the heart – this is known
as tricuspid regurgitation.
Stage IV: One or both babies shows signs of hydrops. This means there is
excess fluid in parts of the baby such as swelling of the skin around the head (scalp
edema), fluid in the abdomen (ascites), fluid around the lungs (pleural effusions)
or fluid around the heart (pericardial effusion). These findings are evidence of
heart failure and are typically seen in the recipient twin.
Stage V: One or both babies have died. The survival of the twins is poorer
when there is progression to a higher stage over time. It has been estimated that
half of patients will progress to a higher stage, 30 percent will remain at the
same stage and 20 percent will improve to a lower stage.2, 3
What is an acardiac twin or twin
reversed arterial perfusion (TRAP) syndrome?
An usual form of TTTS occurs in about one in 15,000 pregnancies. In these monochorionic
twins, one twin develops normally while the other twin fails to develop a heart
as well as other body structures. The abnormal twin is called an acardiac twin.
In these pregnancies, the umbilical cord from the acardiac twin branches directly
from the umbilical cord of the normal twin. Blood flow to the acardiac twin comes
from the normal twin, which is also known as a pump twin. This blood flow is reversed
from the normal direction leading to the name for this condition: twin reversed
arterial perfusion syndrome or TRAP. In some cases the blood flow from the pump
twin to the acardiac twin stops on its own and the acardiac twin stops growing.
In other cases, the flow continues and the acardiac twin continues to increase in
size. This eventually leads to heart failure and polyhydramnios in the pump twin.
Without treatment, more than 50 percent of cases of TRAP will result in the death
of the pump twin.
What is the outcome for Twin-Twin Transfusion Syndrome (TTTS)?
There are a number of ways to treat TTTS, any of which many be the correct method
depending on ultrasound findings, the gestational age of the pregnancy and a couple’s
Left untreated, TTTS prior to 24 weeks’ gestation (six months of pregnancy), 80
percent to 90 percent of cases are associated with the loss of one or both twins.
If one of the twins should die, the blood vessel connections in the placenta can
place the surviving twin at risk for long-term brain damage in as many as one-third
of cases. In general, more advanced stages of TTTS have a worse prognosis than the
earlier stages. When severe TTTS occurs at a very early gestational age (prior to
sixteen weeks or the fourth month of pregnancy), the option of termination of the
pregnancy can be considered due to the grim prognosis.
The various therapies that are available target either the unequal fluid between
the twins’ sacs or interrupt the blood vessel communications between the twins on
the single placenta. The successful outcome of these treatments has been based on
the number of babies that survive, as well as the number of babies who do not have
brain damage. The treatments that are currently available are described below:
Serial amniocentesis involves the removal of the excessive amniotic fluid from the
sac of the recipient twin using a needle that is passed through the maternal abdomen.
The amount of amniotic fluid removed will vary based on the initial volume in the
recipient sac, the gestational age and the development of uterine contractions during
the procedure. As a general rule no more than three liters (approximately two and
one-half quarts) of amniotic fluid is removed at any one time. The procedure is
usually completed within 30 minutes or less. The procedure may temporarily restore
the balance in the amniotic fluid in both twins’ sacs. This technique may be useful
for milder cases of TTTS that occur later in pregnancy.
However, reduction amniocentesis usually requires repeat procedures to be undertaken
every few days to weekly when the fluid returns to high levels. The procedure is
generally not thought to be effective for more advanced stages of TTTS (Stages III
Complications of repeated amniocenteses for the treatment of TTTS include premature
labor with early delivery in 3 percent of cases, premature rupture of the membranes
in 6 percent of cases, infection in about 1 percent of cases, and premature separation
of the placenta from the wall of the womb (called an abruption) in 1 percent of
Pregnancies managed with serial reduction amniocentesis on average deliver
by 29 to 30 weeks of gestation (approximately ten weeks prior to the “due date”).4,
5 If there is progression of TTTS to a more advanced stage, serial amniocenteses
will reduce the success rate for such procedures such as laser (see below). Reported
survival rates have varied from 18 to 83 percent, with a recent study noting that
just over half (56 percent) of severe TTTS cases managed with reduction amniocentesis
will end with at least one infant without brain damage.5 Approximately,
20 percent to 25 percent of the TTTS survivors from pregnancies treated with reduction
amniocentesis have been found to have long-term developmental delay.
Septostomy (also known as microseptostomy)
Septostomy is the creation of a hole in the membrane between the babies’ sacs using
a needle. This causes fluid to move from the amniotic sac with excessive fluid (the
recipient’s sac) into the sac with absent or low fluid (donor’s sac). Since septostomy
is performed with a needle that is used to perform amniocentesis, complications
of infection, premature labor and premature rupture of the membranes are rare. Septostomy
carries the additional potential risk for the hole to become larger between the
two sacs and could even allow the babies to share the same amniotic space if the
entire separating membrane becomes disrupted. This has been reported to occur in
3 percent of septostomies.6 In the worst case scenario, the umbilical
cords of the twins could become entangled, leading to the death of one or both fetuses.
In one large series, survival to birth was 80 percent for at least one twin and
was 60 percent for both twins.6 Patients undergoing septostomy typically
require fewer procedures than those treated with amnioreduction. There is no data
currently available regarding neurologic outcome in survivors of septostomy.
Illustration: Septostomy (also known as Microseptostomy)
Selective laser ablation of the placental anastomotic vessels
In more advanced stages of TTTS (Stage II and higher) laser ablation of the communicating
vessels on the placenta between the twin fetuses can be a curative procedure.
The procedure is performed in an operating room. After the patient’s abdomen has
been washed with an antiseptic and covered with sterile paper drapes, an ultrasound
is performed to determine the appropriate spot to enter the uterus. The skin is
then injected with an anesthetic medication to “numb” the area and deep tissues
directly under the selected site. An anesthesiologist will also administer medications
through an intravenous line to produce sedation. A small skin cut is made to allow
the introduction of a thin hollow tube and needle. The instruments are inserted
under ultrasound guidance into the amniotic sac of the recipient twin. The needle
is removed and a telescope (fetoscope) with a thin fiber to carry the laser energy is then inserted through the hollow tube. The fetoscope is used to look directly
at the blood vessels on the surface of the placenta. Vessels that are found to communicate
between the twins are then closed using laser light energy. At the completion of
the surgery, the extra amniotic fluid in the recipient twin’s sac is removed to
achieve a normal volume. The procedure may take 45 minutes to two hours depending
on the difficulty of the case.
Because the fetoscope requires a larger hole to be made into the amniotic cavity
than would be the case with an amnioreduction or septostomy procedure, laser ablation
is associated with a higher risk of complications such as premature contractions,
premature rupture of the membranes (15 percent to 20 percent of cases), placental
separation (2 percent) and infection. For this reason, special medications to prevent
contractions and antibiotics to prevent infection will be given before and after
the procedure. In addition, laser therapy may be associated with unique risks since
the laser energy may cause certain areas of the placenta or blood vessels on the
surface of the placenta to bleed.
Laser ablation has been shown to result in the survival of at least one twin in
70 percent to 80 percent of cases and both twins in one-third of cases.5,
7, 8 Should one fetus die after the procedure, the likelihood that the
surviving fetus will develop complications is reduced from 35 percent to approximately
7 percent. This is because the babies are no longer sharing blood vessels between
them. In one-third of cases, neither twin will survive. Studies to date have indicated
that approximately 8 percent of survivors following laser ablation will have a long-term
mental handicap. This is approximately half of the rate of problems seen in survivors
treated with amnioreduction.5
Illustration: Laser Ablation of the placental anastomotic vessels
Selective cord coagulation
In some cases, a couple may make the difficult decision to proceed with the purposeful
loss of one twin to save the other twin. This procedure is used when laser ablation
of the connecting vessels is not possible or if one of the twins is so close to
death that laser ablation would likely not be successful. By stopping the flow in
the cord of the dying twin, the other twin can be protected from the consequences
of its sibling’s death. The procedure is performed through the use of a special
forceps that is placed into the amniotic sac of the recipient twin while watching
with ultrasound. The umbilical cord is then grasped and electrical current is applied
to burn (coagulate) the blood vessels in the cord so that the blood flow will stop to this fetus. The communication between the fetuses is definitively ended; however,
this eliminates the chance of survival for one of the twins. Complications of this
procedure include premature delivery and premature rupture of the membranes. Rupture
of the membranes has been reported to occur in about 20 percent of cases. Survival
of the one remaining fetus can be expected in 85 percent of cases.
Illustration: Selective cord coagulation
This procedure is usually reserved for TRAP sequence. The umbilical cord of the
acardiac fetus is usually very short and difficult to see on ultrasound. As a result,
it is often difficult to stop the blood flow into the acardiac fetus by coagulation
of the umbilical cord. For this reason, a major blood vessel in the acardiac fetus
is often targeted as the site for occlusion of blood flow. This can be accomplished
through the use of a radio-frequency ablation catheter. In this procedure, a specialized
needle is passed into the amniotic fluid and then into the body of the acardiac
fetus. A special current is then applied to the needle to burn the area around the
major blood vessel in the abnormal fetus. This will stop the blood flow and allow
the pump twin (normal twin) to no longer have to send blood to the acardiac twin.
Complications of infection, premature contractions and premature rupture of the
membranes can occur as in any needle procedure. In one series, the risk for premature
rupture of the membranes was 8 percent.9 In this same series, the chance
for a successful live birth for the pump twin was 90 percent.
Illustration: Radiofrequency Ablation
What do I do after my physician has made a referral for me to be seen?
You will be scheduled for an ultrasound evaluation with one of our maternal-fetal
medicine specialists at Baylor College of Medicine's Baylor Clinic. The physician will discuss all findings and will review the
treatment options, surgical procedures, prognosis, and recommended follow-up care.
We will be able to answer your questions and concerns at this time.
Next, you will meet with or have a phone conversation with our fetal therapy coordinator
who will be able to answer any further questions that you and your partner may have.
In addition, the fetal therapy coordinator will assist you with any special needs,
including overnight accommodations. You will receive a folder that contains information
you will need for surgery and additional information that you will find helpful.
After your ultrasound and consultation, you will have a pre-operative consultation
with an anesthesiologist, which will be held in the labor and delivery unit of St. Luke's Episcopal Hospital.
You will also have blood samples drawn at that time. A surgery consent form will
be given to you to review. It explains the surgery in terms you can understand.
You will also be given several consent forms for collection of data for an ongoing
study of research to help us better understand the treatment of twin-twin transfusion
syndrome. You will then be discharged home or back to your hotel room.
How do I prepare for surgery?
The night before surgery, you will not be allowed to eat or drink for a defined
amount of time (usually six to eight hours). This is to prevent the risk of vomiting
during surgery. In medical terms, this is known as "NPO" (nothing by mouth).
We will give you a time to come to St. Luke's Episcopal Hospital on the day of surgery.
Your family may come with you, but will be asked to wait for you in one of our labor
or waiting rooms during the surgery. An intravenous line (IV) will be inserted by
needle stick to give you fluids and medications during surgery. An ultrasound will
be performed prior to going to the operating room to confirm that both twins are
alive. One of the specially trained nurses that will be assisting in surgery and
an anesthesiology resident will accompany you to surgery.
What can I expect during surgery?
You will be transported to the operating room where you will be asked to move on
to the operating table. You will be covered with a warm blanket and a pillow will
be placed under your knees to keep you comfortable during surgery. You may be rolled
to your left side to keep your uterus from causing your blood pressure to fall.
A belt will be placed across your legs to prevent you from sliding off the operating
room table. Your abdomen (belly) will be cleaned with an iodine solution (let your
nurse know if you are allergic to iodine). Then you will be covered with sterile
paper drapes. The top of the drapes will be attached to a pole so that you do not need to watch the procedure. Medication will be given through your IV to relax you.
Surgery is performed under local anesthesia, meaning you are awake but relaxed,
and your abdomen is numbed where the instrument is inserted. An anesthesiologist
will stay with you throughout the procedure. You will be given additional medication
for discomfort as needed. On rare occasions, general anesthesia, meaning you are
put to sleep, may be used. During surgery, one or two small incisions approximately
one-tenth of an inch long will be made on the abdomen. These incision(s) are small.
You will have short pieces of specialized tape (steri-strips) placed on your skin
to close this incision at the end of the procedure.
What can I expect after surgery?
Following surgery, you will be taken to the recovery room in labor and delivery
or a labor room where you and your fetus or fetuses will be closely monitored. Your
abdomen will be a little tender or sore once the local anesthetic wears off. You
may be given medications after surgery to relax the uterus and stop any contractions.
The pain and discomfort after surgery is usually minimal. If needed, pain relief
medicine is available. Your spouse or other support person may remain with you in
your room. Following surgery, you may have food as tolerated. You will be admitted
to the hospital for an overnight stay. That night, activity is restricted to bathroom privileges only, but this depends upon your specific condition. You will undergo
an ultrasound the day after surgery to determine how the babies are doing.
What can I expect after I am discharged
You will then be discharged home to the care of your primary obstetrician
and/or your referring maternal-fetal medicine specialist. Your instructions will
include bed rest with bathroom privileges for seven days after the surgery, with
a gradual increase in activity. We will also ask that you get a thermometer and
take your temperature three times per day. You should notify your primary obstetrician
for any increase above 100.4°F of an oral temperature. The site of the surgery can
get wet in a shower within 24 hours of the procedure. You can remove the steri-strips
over the incision yourself one week after the surgery.
After four weeks you can resume normal activity based on your pregnancy condition
and the comfort level of your primary obstetrician. Weekly ultrasounds are recommended
for the next month. After that time, if all is going well, ultrasounds are performed
as directed by your doctor.
Although you are returning home, we will continue to follow your pregnancy closely
through our care coordinator. Please make arrangements with your doctor to forward
your ultrasound reports and any other pertinent information to us. We also ask that
you inform your obstetrician and labor nurse that we would like to have your placenta
sent back to us after you delivery. This information is useful to further our knowledge
and will assist in the future treatment of patients with TTTS.
Social services and pastoral care are available for all our patients and their families.
If you would like to see them at any time, you need only to request it and they
can be contacted. We are sensitive to the psychological, social and spiritual needs
of our families. We will provide any support that is necessary. Please contact us
if you have any questions, concerns or special requests. For our out of town patients,
we realize that traveling may be difficult or stressful and want you to know that
we will do everything we can to accommodate your special needs and schedule.
1. Quintero RA, Morales WJ, Allen MH, Bornick PW, Johnson PK, Kruger M. Staging
of twin-twin transfusion syndrome. J Perinatol 1999; 19:550-5.
2. Taylor MJ, Govender L, Jolly M, Wee L, Fisk NM. Validation of the Quintero staging
system for twin-twin transfusion syndrome. Obstet Gynecol 2002; 100:1257-65.
3. Dickinson JE, Evans SF. The progression of disease stage in twin-twin transfusion
syndrome. J Matern Fetal Neonatal Med 2004; 16:95-101.
4. Mari G, Roberts A, Detti L, et al. Perinatal morbidity and mortality rates in
severe twin-twin transfusion syndrome: results of the International Amnioreduction
Registry. Am J Obstet Gynecol 2001; 185:708-15.
5. Senat MV, Deprest J, Boulvain M, Paupe A, Winer N, Ville Y. Endoscopic laser
surgery versus serial amnioreduction for severe twin-to-twin transfusion syndrome.
N Engl J Med 2004; 351:136-44.
6. Moise KJ, Jr., Dorman K, Lamvu G, et al. A randomized trial of amnioreduction
versus septostomy in the treatment of twin-twin transfusion syndrome. Am J Obstet
Gynecol 2005; 193:701-7.
7. Hecher K, Plath H, Bregenzer T, Hansmann M, Hackeloer BJ. Endoscopic laser surgery
versus serial amniocenteses in the treatment of severe twin-twin transfusion syndrome.
Am J Obstet Gynecol 1999; 180:717-24.
8. Quintero RA, Dickinson JE, Morales WJ, et al. Stage-based treatment of twin-twin
transfusion syndrome. Am J Obstet Gynecol 2003; 188:1333-40.
9. Lee H, Wagner A, Bobert B, et al. Radiofrequency ablation for TRAP sequence.
Am J Obstet Gynecol 2005; 191:S18.