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Twin-to-Twin Transfusion Syndrome

Also known as TTTS, Twin-to-twin transfusion syndrome. Most often the second trimester; screened from 16 weeks in shared-placenta twins.

Twin-twin transfusion syndrome, or TTTS, can happen only in identical twins who share a single placenta. Abnormal blood-vessel connections in that shared placenta let blood flow unevenly between the twins, so one gets too much and the other too little. It is serious, but two things change the odds: it is caught early through regular ultrasounds done specifically to look for it, and there is an effective treatment, laser surgery, that corrects the underlying problem. This page explains how TTTS happens, how it is found and staged, and what treatment can do. If you are carrying shared-placenta twins, this is exactly what that extra monitoring is for.

Know the warning signs When to call a provider
How common
10% to 15%
Of identical twins who share one placenta; it cannot occur in separate-placenta twins.
When
Usually second trimester
Screened for every two weeks from about 16 weeks in shared-placenta twins.
Urgency
Specialist care
Needs prompt referral to a fetal center; some stages need urgent treatment.
Outlook
Much improved with treatment
With laser surgery, at least one twin survives in roughly 85% to 90% of cases.

Understanding it

What is twin-to-twin transfusion syndrome?

Twin-twin transfusion syndrome is unique to monochorionic twins, identical twins who share one placenta. In a shared placenta, the two babies' blood vessels are connected. Normally those connections are balanced, with blood flowing evenly in both directions. In TTTS the balance is lost: blood shunts from one twin, the donor, toward the other, the recipient, through the connecting vessels.

That imbalance hits both twins in opposite ways. The donor, giving away blood, becomes smaller, makes less urine, and ends up with too little amniotic fluid, sometimes so little it appears shrink-wrapped against the wall of the womb. The recipient, taking in too much blood, makes too much urine, is surrounded by excess fluid, and can develop strain on its heart from handling the extra volume. It is a single problem, unequal sharing, with two very different faces.

TTTS develops in about 10% to 15% of shared-placenta twin pregnancies, most often in the second trimester. It usually causes the mother no symptoms, which is exactly why these twins are scanned every two weeks from around 16 weeks. Those surveillance ultrasounds hunt for the tell-tale sign: one twin with too little fluid and the other with too much. When symptoms do appear, they come from the rapid buildup of fluid around the recipient, a belly that suddenly feels much larger, tight, or has gained weight quickly.

What changes the outlook is treatment. For significant TTTS, a procedure called fetoscopic laser surgery seals the abnormal vessel connections in the placenta, separating the two circulations and stopping the unequal sharing. Done at a specialist fetal center, it has transformed the picture: untreated advanced TTTS carries a very high risk of losing one or both twins, while with laser treatment at least one twin survives in the large majority of cases. Milder, early-stage TTTS is sometimes watched closely rather than treated right away.

Symptoms

What are the symptoms?

TTTS usually causes the mother no symptoms and is found on surveillance ultrasound. When symptoms occur, they come from the rapid fluid buildup around the recipient twin.

Early signs

  • Usually none; detected on the routine every-two-week ultrasound in shared-placenta twins
  • A belly that suddenly feels much larger or is measuring big for dates
  • Abdominal tightness, pressure, or pain
  • Rapid weight gain over a short time
  • A sense of breathlessness from the enlarging uterus

Emergency signs — call 911

  • Sudden, rapid enlargement of your belly with tightness or pain, in a known shared-placenta twin pregnancy
  • Abrupt weight gain over a few days
  • Signs of preterm labor: regular contractions, pelvic pressure, or leaking fluid
  • Any concern in a monochorionic twin pregnancy, which warrants prompt evaluation at your fetal center

Why it matters

Risks to you and baby

Risk to both twins

Untreated, significant TTTS endangers both babies: the donor from too little blood and fluid, the recipient from volume overload and heart strain. Advanced untreated TTTS carries a very high risk of losing one or both twins, which is why early detection matters so much.

Preterm birth

The rapid fluid buildup around the recipient overstretches the uterus, raising the chance of preterm labor and early water breaking. Treatment itself also carries some risk of preterm delivery.

Heart strain in the recipient

Handling the extra blood volume strains the recipient twin's heart, which in advanced cases can progress to heart failure and fluid buildup called hydrops. This is one of the features used to stage the condition.

Effects that can persist

Even after successful treatment, the twins may need monitoring for growth, heart, and, less commonly, neurological effects. The specialist team follows this through the pregnancy and after birth.

How it's found

How is it diagnosed?

TTTS is diagnosed by ultrasound showing a striking difference in amniotic fluid between the twins, and is graded in stages that guide treatment.

Surveillance ultrasound

The diagnostic sign is fluid discordance: the donor twin with too little fluid (a deepest pocket under 2 centimeters) and the recipient with too much (over 8 centimeters). This is why shared-placenta twins are scanned every two weeks.

Doppler and bladder assessment

Doppler studies of blood flow, and whether each twin's bladder is visible, help confirm the diagnosis and judge severity.

Quintero staging

TTTS is graded from Stage I (fluid difference only) to Stage V (loss of a twin), based on fluid, bladder filling, blood flow, and heart function. The stage guides whether to treat and how.

Set by the placenta type on an early scan, then screened every two weeks from about 16 weeks. Diagnosis is confirmed whenever the fluid discordance appears.

Treatment

How is it managed?

Treatment depends on the stage. Early, mild TTTS may be watched closely, while significant TTTS is treated with laser surgery at a specialist fetal center to correct the underlying vessel problem.

Treatment paths

For significant TTTS, roughly Stage II and above, the standard treatment is fetoscopic laser photocoagulation: through a thin scope, the surgeon finds and seals the abnormal connecting vessels in the placenta, separating the two circulations so the unequal sharing stops. Draining excess fluid (amnioreduction) is an alternative or additional measure in some situations. Early, mild TTTS is often managed with close surveillance, since some cases stay stable or improve.

Monitoring

Care is led by a maternal-fetal medicine team at a fetal center, with frequent ultrasounds before and after any procedure to track both twins.

Can it recur?

TTTS can only occur in a shared-placenta twin pregnancy, which is uncommon, so it does not recur in the usual sense; a future pregnancy would again need to be identical twins sharing a placenta. Within a treated pregnancy, TTTS can occasionally persist or reverse from residual vessels, which is why the team keeps monitoring after treatment.

What you can do

Can it be prevented?

  • Know the placenta type early

    Determining whether twins share a placenta, best done on a first-trimester ultrasound, is what flags a pregnancy as at risk for TTTS and puts the right monitoring in place.

  • Keep every surveillance scan

    Because TTTS usually has no symptoms, the every-two-week ultrasounds from 16 weeks are how it is caught early, when treatment works best. Do not skip them.

  • Be cared for at or near a fetal center

    TTTS treatment, especially laser surgery, is done at specialist fetal centers. Knowing where yours is, and being connected to it, means faster care if TTTS develops.

  • Report sudden changes

    A belly that suddenly feels much larger, tight, or has gained weight quickly should prompt an immediate call, as it can signal the rapid fluid buildup of TTTS.

Who is more at risk

Risk factors

Sharing a single placenta
The one essential risk factor. TTTS occurs only in monochorionic twins, identical twins who share one placenta, and cannot happen otherwise.
Develops in 10% to 15% of shared-placenta twin pregnancies.
The pattern of placental vessels
Within shared-placenta twins, the number and balance of the connecting vessels, and how evenly the placenta is shared, determine which pregnancies develop TTTS. This cannot be controlled or predicted in advance.
No maternal or lifestyle cause
TTTS is not caused by the mother's age, diet, activity, or anything she did. It is a feature of the shared placenta's blood vessels, which is important to understand, because it is no one's fault.

Do not wait

When to call your provider or 911

  • In a known shared-placenta twin pregnancy, call promptly if your belly suddenly feels much larger, tight, or painful.
  • Report abrupt weight gain over a few days, which can reflect rapid fluid buildup.
  • Seek care for signs of preterm labor: regular contractions, pelvic pressure, or leaking fluid.
  • Keep all of your every-two-week surveillance ultrasounds, and go to any extra scans your team recommends.
  • Contact your maternal-fetal medicine team with any concern; TTTS is managed by specialists and warrants prompt evaluation.

Talking to your team

Questions to ask your provider

  • Do my twins share a placenta, and am I being screened for TTTS?
  • How often will I have ultrasounds, and starting when?
  • If TTTS develops, what stage would need treatment?
  • Is laser surgery available here, or would I be referred to a fetal center?
  • What symptoms should make me call right away?
  • What are the survival rates with treatment for a case like mine?

Keep reading

More on twin-to-twin transfusion syndrome

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Good to Know

Twin-to-Twin Transfusion Syndrome FAQs

Common questions about twin-to-twin transfusion syndrome, answered.

What is twin-twin transfusion syndrome?

It is a complication of identical twins who share one placenta, in which abnormal blood-vessel connections let blood flow unevenly between them. One twin, the donor, gives away blood and ends up small with too little fluid; the other, the recipient, gets too much blood and fluid and can develop heart strain. It affects about 10% to 15% of shared-placenta twins and is treatable.

Can it happen in any twin pregnancy?

No. TTTS can only occur in identical twins who share a single placenta, called monochorionic twins, because it depends on connected blood vessels between them. Fraternal twins, and identical twins who each have their own placenta, cannot develop it. This is why determining the placenta type early is so important.

How is it found if there are no symptoms?

Through regular ultrasounds. Because TTTS usually causes no symptoms, shared-placenta twins are scanned every two weeks starting around 16 weeks specifically to look for it. The scan checks for the key sign: one twin with too little amniotic fluid and the other with too much.

What is the treatment?

For significant TTTS, the standard treatment is fetoscopic laser surgery, in which a surgeon uses a thin scope to seal the abnormal vessel connections in the placenta, separating the two circulations and stopping the unequal sharing. It is done at a specialist fetal center. Draining excess fluid is an alternative in some cases, and mild, early TTTS may be watched closely.

Will both my babies survive?

Treatment has greatly improved the odds. Untreated advanced TTTS carries a very high risk of losing one or both twins, but with laser surgery, at least one twin survives in roughly 85% to 90% of cases, and both survive in around two-thirds. Outcomes depend on the stage at treatment, which is why early detection matters so much.

Did I do something to cause this?

No, absolutely not. TTTS is caused by the pattern of blood vessels in the shared placenta, set by how the placenta formed, not by anything you did, ate, or could have prevented. It is not related to your age, activity, or diet. Understanding this can help, because parents often search for a reason and there simply is not one.

What are the stages?

TTTS is graded from Stage I to Stage V using the Quintero system, based on the fluid difference, whether each twin's bladder is visible, the blood flow patterns on Doppler, and the recipient's heart function, with Stage V meaning the loss of a twin. The stage guides whether to treat and how, with early stages sometimes watched and more advanced stages treated with laser.

Could it happen in my next pregnancy?

Only if you again had identical twins sharing a placenta, which is uncommon, so TTTS is not something that recurs in the usual sense. There is no way to predict or prevent the vessel connections that cause it. If you did have another shared-placenta twin pregnancy, it would be monitored the same way from the start.

Medically Reviewed · Updated

Reviewed by Dr. Grace Lin, MD, FACOG · OB-GYN

Obstetrician-gynecologist focused on reproductive and sexual health for women: pregnancy, BV, yeast, trichomoniasis and HPV/cervical screening. Our editorial guidelines →

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