Everything You Need to Know About Conjoined Twins

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Conjoined twins occur when identical twins have fused bodies before they are born. This condition has also been known as "Siamese twins," named after the first world-recognized set of conjoined twins born in 1811 in Siam, present-day Thailand. "Siamese twins" is now an inappropriate, outdated term.

Unfortunately, the majority of conjoined twins die before birth or shortly thereafter. The prognosis for separating conjoined twins depends on where their bodies are connected.

This article discusses the causes, types, and diagnosis of conjoined twins, as well as separation surgery.

How Many Twins Are Born Conjoined?

The exact occurrence of conjoined twins is not known, but it's estimated to be between 1 in 30,000 to 1 in 200,000.

Asian twin boys

Edwin Tan / Getty Images


Conjoined twins are a rare occurrence, which makes it more difficult to determine the exact cause. However, there are two theories about the possible causes of conjoined twins.

Conjoined twins are always identical twins. When identical twins form in utero, one fertilized egg, called an embryo, splits into two separate individuals. The main theory for conjoined twins is that this splitting process isn't complete.

The second theory for conjoined twins suggests that two separate embryos fuse together while they are developing.

Types of Conjoined Twins

Conjoined twins are classified by the area of the body where they are connected.

Most common types are:

  • Thoracopagus: Babies lie face to face and share a breastbone, diaphragm (breathing muscle), and upper abdominal wall. In some cases, the babies might share a heart. About 75% of conjoined twins fall into this category.
  • Pygopagus: Babies are facing away from each other and joined at the buttocks and the area between the anus and genitals. They sometimes share the bottom of the spinal cord and often share a rectum and anus, as well as genitalia. This category represents around 20% of conjoined twins.
  • Omphalopagus: This is a subgroup of thoracopagus twins, with the babies face to face, sharing an anterior abdominal wall from the bottom of the breastbone to the belly button. These twins often share a liver. This is the least complicated type of conjoined twins.

Least common types are:

  • Ischiopagus: Babies are joined at a single pelvis. These twins might have all four legs, but two of their legs are often fused. These twins share a single colon. Less than 5% of conjoined twins fall into this category.
  • Craniopagus: This is the least common type of conjoined twins, representing less than 2%. These twins have fused skulls. Their brains may or may not be connected.


Conjoined twins are typically diagnosed early on in pregnancy, with the earliest reported diagnosis at eight weeks of gestation. Conjoined twins are diagnosed with various types of imaging, including:

  • Fetal ultrasound: High-frequency sound waves that produce an image, which can be 3D (three dimensional) or 4D (four dimensional)
  • Fetal MRI: Magnetic resonance imaging (MRI) that provides more detailed images for body parts, such as internal organs
  • Ultrasound fetal MRI: Combines ultrasound imaging with MRI imaging
  • Fetal echocardiogram: Ultrasound imaging of the heart to assess for abnormalities

Pregnancy Risks

Carrying conjoined twins can cause additional pregnancy risks such as preterm birth and delivery by cesarean section, or C-section, and you'll likely be followed by a doctor who specializes in high-risk pregnancies.

Conjoined Twin Separation Surgery

There are many factors that go into a decision to perform separation surgery, including the extent of shared organs and the surgeries that will be required to reconstruct body parts after separation surgery. In addition, there are ethical considerations, as surgery can result in the death of one or both of the babies.

Not all conjoined twins are candidates for surgery, such as when the babies share a heart. Separation surgery is a very complex procedure that requires multiple health care professionals with varying areas of expertise, including:

  • Neurosurgeons: Medical doctors who treat the brain, spine, and nervous system
  • Cardiologists: Medical doctors who treat heart diseases and abnormalities
  • Neonatologists: Pediatric medical doctors who specialize in the care of newborn infants, especially those who are ill or premature
  • Maternal-fetal medicine specialists (perinatologist): Medical doctors who specialize in caring for women having complicated or high-risk pregnancies.
  • Advanced practice nurses: A registered nurse with post-graduate nursing education and training

Preparation for separation surgery often includes putting tissue expanders under the skin to stretch it out to provide skin for covering the area that is separated during surgery.

About 60% of separation surgeries are successful.

Conjoined Twins Successfully Separated

Twins who are joined at the head typically have a poor prognosis. However, in October 2020, a pair of 9-month-old craniopagus twins were successfully separated at the University of California, Davis, Children's Hospital. The procedure lasted 24 hours.


Conjoined twins occur very rarely. They are categorized by the area of the body that is connected. Most commonly, conjoined twins are joined at the chest. The prognosis for conjoined twins is not good. Many do not survive in utero or they die shortly after birth. However, separation surgery can be performed successfully in some cases. Conjoined twins are typically diagnosed early on in pregnancy with ultrasound.

A Word From Verywell

Receiving the news that you're carrying conjoined twins can be overwhelming. Working closely with your care team and learning about your condition can help you know what to expect during your pregnancy.

Frequently Asked Questions

  • What are the potential complications of delivering conjoined twins?

    Conjoined twins are always delivered via C-section and are often born early.

  • Are conjoined twins always identical?

    Because conjoined twins develop from one fertilized egg, they are always identical.

  • How long do conjoined twins live?

    The majority of conjoined twins die in utero or shortly after birth. However, some conjoined twins live into adulthood. The oldest conjoined twins on record lived to be 68 years old.

10 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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  4. Children's Hospital of Philadelphia. Conjoined twins.

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  10. ABC News. World's longest-surviving conjoined twin brothers die at 68.

By Aubrey Bailey, PT, DPT, CHT
Aubrey Bailey is a physical therapist and professor of anatomy and physiology with over a decade of experience providing in-person and online education for medical personnel and the general public, specializing in the areas of orthopedic injury, neurologic diseases, developmental disorders, and healthy living.