Conjoined Twins

Conjoined twins are caused by two embryos that are joined together during fertilization, resulting in twins that are physically connected, most commonly at the abdomen, chest or head. Conjoined twins are rare. Complications can be reduced with close management from healthcare professionals.


Twin girls joined at the shoulder share a liver and diaphragm.
Twin conjoined twins girls joined at the shoulder.

What are conjoined twins?

When two babies are physically connected to each other at birth, they're called conjoined twins. Experts don’t know exactly what causes the condition. But it likely involves splitting or fusion of very early-stage embryos soon after fertilization. All conjoined twins are identical, and about two-thirds are assigned female at birth (AFAB).


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How common are conjoined twins?

Conjoined twins are rare. It's estimated to occur only once in every 50,000 pregnancies. Because the conjoined anatomy is sometimes incompatible with life, many conjoined embryos will not survive. Up to 60% of conjoined twins are stillborn (not alive at birth) or die shortly after delivery.

Are conjoined twins always identical?

Yes. Conjoined twins are always identical. They’re also more likely to be female.


Symptoms and Causes

What causes conjoined twins?

There are two theories of what causes conjoined twins:

  • Fission: An early embryo, comprising a small sphere of identical cells, splits into two spheres but doesn’t separate completely. The two spheres each develop independently into the conjoined twins.
  • Fusion: An identical twin pregnancy contains separate early twin embryo “spheres” that merge together and join at a random point of connection.

Both of these theories help explain how conjoined twins might form during the first 12-14 days of fertilization. But scientists still don’t know why this happens.

What are the types of conjoined twins?

Conjoined twins are identified by specific terms. These terms describe the point of connection. Twins may be conjoined at the:

  • Abdomen (omphalopagus).
  • Chest (thoracopagus).
  • Top of head down to the belly button, facing each other (cephalopagus).
  • Head only (craniopagus).
  • Pelvis, facing each other (ischiopagus).
  • Pelvis, side-to-side (parapagus).
  • Rump-to-rump (pygopagus).
  • Vertebral column (rachipagus).


Diagnosis and Tests

How are conjoined twins diagnosed?

Conjoined twins can be identified as early as 12-weeks prenatally through imaging tests, including:

If imaging tests show that twins are conjoined, healthcare providers will try to predict potential complications. Then they’ll make a plan to address them at the time of birth.

Management and Treatment

Are there complications during pregnancy with conjoined twins?

It is very common for birthing parents to experience premature labor with conjoined twins. With early detection, the pregnancy can be managed to prevent pre-term labor and allow the twins to develop normally.

Delivery by C-section is the most common way to manage this situation.

Can conjoined twins be separated?

Sometimes. Successful management usually involves several medical and surgical specialists. Sophisticated imaging techniques and other tests can help sort out the anatomy and physiology of both twins. The decision to attempt separation or not can bring up difficult ethical questions. These questions are usually discussed at length between the parents, the medical team, and often clergy and medical ethicists.

In some cases, it’s possible to separate conjoined twins. The procedure to separate twins always takes extensive planning by a team of highly experienced surgeons.

After conjoined twins are born, they are grouped into one of three categories:

  • No separation: If separation would pose life-threatening risks to the twins or result in extreme disabilities, surgical separation would only be attempted after consideration of all quality-of-life issues.
  • Immediate separation: Sometimes emergency surgical separation is required to save the life of one and/or both twins.
  • Delayed separation: When the twins are stable from a health standpoint, your medical team may delay separation surgery. This gives the twins time to grow and develop — and it increases their chance of survival. Many healthcare providers feel that surgical separation should take place when conjoined twins are 9 to 12 months old. This way, they can be separated before the condition takes a psychological toll. Twins who experience delayed separation have a higher survival rate (80%) over twins who need immediate separation.


Can conjoined twins be prevented?

Since the cause of conjoined twins is unknown, there’s no way to prevent conjoined twins from occurring during pregnancy.

Outlook / Prognosis

What is the prognosis for conjoined twins?

Survival rates and quality of life for conjoined twins depend entirely on how the two babies are connected. Here are the survival rates for each type:

  • Abdomen (omphalopagus): 82%.
  • Rump-to-rump (pygopagus): 68%.
  • Pelvis, facing each other (ischiopagus): 63%.
  • Chest (thoracopagus): Poor.
  • Pelvis, side-to-side (parapagus): Poor.
  • Top of head to bellybutton (cephalopagus): Poor.
  • Head only (craniopagus): Poor.
  • Vertebral column (rachipagus): Cannot be separated.

If separation surgery is attempted, only 60% of surgically separated conjoined twins survive. Healthcare providers can manage complications to improve the outlook for conjoined twins.

Can conjoined twins lead a healthy life?

Depending on how the twins are joined, conjoined twins may live fully functioning lives thanks to advancements in surgical separation. Some conjoined twins who can't be separated can still enjoy a good quality of life if they’re closely monitored.

Living With

During pregnancy, when will I know if I have conjoined twins?

Conjoined twins can be identified as early as 12 weeks into your pregnancy. Your provider can usually confirm this around the 20-week mark.

What questions should I ask my healthcare provider?

  • Where are my twins connected and what organs are shared?
  • Will separation of my twins be possible?
  • Will immediate separation be necessary?
  • How can premature labor be prevented?
  • Is there a team of specialists available to monitor and treat my babies before and after they’re born?

A note from Cleveland Clinic

The very last thing parents want to hear is that there’s something unexpected going on with their unborn children. Since we don’t know why conjoined twins happen, the rare condition can’t be prevented. After diagnosis, you’ll want to start working with a team of highly qualified medical professionals who can monitor the development of your twins, help you develop a birth plan and start to discuss surgical interventions. Treatment of conjoined twins depends on where they’re conjoined and the extent of organ sharing. But there are surgical and medical options for many conjoined twins who survive birth. It’s normal to have a range of intense feelings, including fear, anger, guilt and sadness. Remember that your healthcare team is there to support you each step of the way.

Medically Reviewed

Last reviewed on 04/26/2022.

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