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Unknown: Conjoined Twins

Unknown: Conjoined Twins

Causes and Symptoms
The Future
For more information


Conjoined twins are identical twins in which the two embryos fail to separate completely before birth. It is thought that the condition results from incomplete splitting after the twelfth day of fetal development. If a fertilized human egg divides into two embryos before the twelfth day, the identical twins will be born normally as two separate infants. The longer the delay in the separation of the two embryos, the more complicated the connections between the conjoined twins are likely to be.

Conjoined twins can be categorized in several different ways. One distinction that is often made is between symmetrical (equal) conjoined twins, in which both infants are well developed; and asymmetrical (unequal) conjoined twins, in which an incomplete twin is joined to a fully developed sibling. Conjoined twins are also classified according to the points at which their bodies are joined. This system goes back to a French biologist named Étienne Geoffroy Saint-Hillaire (1772–1844), who based his terms on the formal medical words for parts of the body. These categories are listed in the next section.


Conjoined twins may share tissue, organ systems, or both, depending on the points at which their bodies are fused:

  • Thoracopagus: 40 percent of conjoined twins are joined at the chest. These twins always share a heart.
  • Omphalopagus: This term refers to twins joined at the abdomen, and these account for 34 percent of cases. These twins often share a liver and digestive system.
  • Pygopagus: The twins are joined back to back at the buttocks and account for 18 percent of cases. These twins sometimes share the lower part of the digestive tract or the genital organs.
  • Ischiopagus: The twins are joined at the pelvis or lower spine with the lower half of the bodies fused. They have four arms but may have between two and four legs. Six percent of cases fall into this group.
  • Craniopagus: Two percent of conjoined twins are fused at the head but the bodies are separate. In some cases these twins share part of the brain as well as the skull.
  • Parasitic twins: This term is used to describe cases in which one twin is much smaller than the other and depends on the larger twin for survival. Some researchers estimate that 10 percent of conjoined twins fall into this category.


Estimates vary somewhat, but most researchers think that conjoined twins occur once in every 33,000–165,000 births; however, 40–60 percent of conjoined twins are stillbirths. In the United States, one in every 200,000 live births is a set of conjoined twins.

Male conjoined twins are more likely to be stillbirths. Conjoined twins that are born alive are three times more likely to be females.

Conjoined twins are more common in India and Africa than in Europe or North America. The reasons for this difference are not known.

Chang and Eng

Chang and Eng Bunker (1811–1874) were conjoined twins born in 1811 in Thailand (formerly called Siam), for whom the term “Siamese twins,” a term that is no longer culturally accepted, was coined. Joined at the breastbone by a band of cartilage, their vital organs functioned independently; they could have been easily separated if modern surgical techniques and equipment had been available.

Like some other nineteenth-century conjoined twins, Chang and Eng were discovered by a British business promoter in 1829 and exhibited as a medical curiosity. They later joined P.T. Barnum's traveling circus. They left the circus in 1839, settling in North Carolina. Taking the surname Bunker, they became U.S. citizens and married two sisters in 1843. The wives did not get along, so the brothers set up separate households, spending three days each week in each home. Chang had ten children by his wife; Eng had eleven.

The twins' health declined in the early 1870s. Chang died on January 17, 1874; Eng died an hour later. They are subjects of several literary works, the first was a short story by Mark Twain

called “The Siamese Twins.” With characteristic humor, Twain suggested that the twins had fought on opposite sides during the Civil War, and that one was fifty-three years old while his twin was only fifty-one.

Causes and Symptoms

Identical twins develop when a fertilized human egg splits into two separate embryos during the first twelve days following conception. If the split occurs after the twelfth day, the twins will not separate fully from each other and will be conjoined at birth. The cause of most cases of delayed splitting is not known; no genes have yet been identified that lead to conjoined twins. There have been, however, about ten cases reported in which conjoined twinning occurred after the mother had

been exposed to a drug given to stimulate ovulation or a drug used to treat fungal infections.

The symptoms of conjoined twins depend on the location of the fusion between the two twins and whether the twins are symmetrical (equal) or not. In many cases the twins die shortly after birth from heart failure, lung failure, or obstruction in the digestive tract.


In some cases the doctor may suspect the presence of conjoined twins early in the pregnancy by finding that the mother's uterus is larger than expected and hearing two fetal heartbeats when listening through a stethoscope. Conjoined twins can be diagnosed by ultrasound as early as the eighth week of pregnancy. Magnetic resonance imaging (MRI) can be used to identify which organ systems are shared between the twins. Computed tomography (CT) scans are primarily useful in evaluating the bony structures of twins fused at the hips or pelvis.

Electrocardiograms (ECGs) and electroencephalograms (EEGs) may be used to evaluate the extent of shared heart or brain function and to determine whether surgical separation of the twins is possible.


The treatment of conjoined twins is highly individualized. It almost always requires complicated surgery in one of a small number of medical centers (three in the United States, one in the United Kingdom, one in New Zealand, and one in South Africa). Doctors classify conjoined twins for treatment in one of three categories: those who will die shortly after birth; those who require immediate emergency surgery; and those who will survive until they are old enough for surgery to have a higher chance of success (usually six to twelve months). Twins who share a heart or brain usually cannot be separated without causing the death of both twins.

Conjoined twins are usually delivered by cesarean section rather than waiting for the mother's due date. The operation is usually scheduled for two to four weeks before the due date. Surgery to separate the twins may be performed immediately after delivery if one or both twins have a life-threatening emergency. These operations are complicated and may take as long as thirty-five hours to complete. Two complete

surgical teams are required to care for the twins after the separation is complete. In most cases, conjoined twins who survive separation will need further surgery at intervals during childhood.


The prognosis of conjoined twins is often poor. Between 40 and 80 percent of twins who need emergency surgery after birth die in intensive care following the operation. In some cases, particularly those involving parasitic twins, the parents must make the painful decision to allow one of the twins to die if the other is to have any chance of survival. Conjoined twins who are healthy enough to have separation surgery postponed until they are older have a survival rate of 80 percent.

It is possible for conjoined twins who are not separated to have productive and satisfying lives. One set of conjoined twins in Minnesota completed high school in 2008 and obtained a driver's license. One twin in a set of conjoined sisters has made a career as a country music singer. Conjoined twins who are not separated, however, have shortened life expectancies; most pairs die in their twenties or early thirties. As of 2008, the oldest known living set of conjoined twins was a pair of brothers in Ohio born in 1951.


There is no known way to prevent conjoined twins other than avoidance of the small number of drugs that have been associated with conjoined twinning in a few cases. The condition is considered to be sporadic (occurring at random); parents do not have an increased risk of having a second set of conjoined twins in a later pregnancy.

The Future

The widespread use of ultrasound and other imaging techniques has led to improved prenatal identification and diagnosis of conjoined twins. It is possible that the greater numbers of conjoined twins that are identified before birth will help researchers understand the causes of this type of twinning and discover better methods of surgical separation and treatment for conjoined twins.

SEE ALSO Spina bifida


Identical twins : Twins that develop from a single fertilized egg that divides to form two separate embryos.

Prenatal : Before birth.

Sporadic : Occurring at random.

Stillbirth : The birth of a baby that has died before or during delivery.

For more information


Jensen, Dean. The Lives and Loves of Daisy and Violet Hilton: A True Story of Conjoined Twins. Berkeley, CA: Ten Speed Press, 2006. The Hiltons (1908–1969) were conjoined sisters born in England who became vaudeville stars in the 1930s.

Landau, Elaine. Joined at Birth: The Lives of Conjoined Twins. New York: Franklin Watts, 1997.

Quigley, Christine. Conjoined Twins: An Historical, Biological, and Ethical Issues Encyclopedia. Jefferson, NC: McFarland and Co., 2003.


Chichester, Page. “A Hyphenated Life.” Blue Ridge Country Magazine, March 2004. Available online at (accessed July 24, 2008). This is an article about Chang and Eng Bunker, with several illustrations and photographs of the twins.

Santos, Fernanda. “A Fairy-Tale Ending Eludes Separated Twins.” New York Times, November 4, 2007. Available online at (accessed July 24, 2008). This is a report about the developmental difficulties confronting conjoined twins who were separated in 2003.

“Sisters' Hope: Conjoined Twins.” BBC News, September 21, 2005. Available online at (accessed July 24, 2008). This is an article about conjoined sisters from Pennsylvania, one of whom is a country music singer.


Mayo Clinic. Conjoined Twins. Available online at (updated November 21, 2006; accessed July 23, 2008).

National Library of Medicine. From ‘Monsters’ to Modern Medical Miracles: Selected Moments in the History of Conjoined Twins from Medieval to Modern Times. Available online at (updated September 19, 2007; accessed July 23, 2008).

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