Conjoined Twins

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 development. If a fertilized human egg divides into two embryos before the twelfth day, the identical twins are born normally as two separate infants. Conjoined twins were once referred to as Siamese twins named after Chang and Eng Bunker (1811–1874), a pair of male conjoined twins who were born in the Kingdom of Siam (now Thailand). The term is no longer considered appropriate.

Meet the Schappell Sisters

George (born Dori) and Lori Schappell are conjoined twins born in Reading, Pennsylvania, in 1961. They are craniopagus conjoined twins who cannot be separated because they share about 30 percent of their brain tissue. Dori disliked having a name that rhymed with her sister's, so she changed it first to Reba and then, in 2007, to George.

The Schappell's are an example of the physical differences that can exist between conjoined twins. Lori is physically healthy, but George was born with spina bifida * . It was this physical condition that caused the them to be removed from their parents by court order in the early 1960s and placed in an institution for the mentally disabled even though neither had a learning impairment. After they became adults, the Schappell's fought to be released from the institution. They were able to attend college after their release.

The twins took turns working while the other attended school or began a career. George worked in a hospital while Lori finished college. Lori worked in a laundry and helped to manage George's career as a country music singer. George (who still called herself Reba when she began to sing) performed in Germany and Japan as well as in the United States, and won the L.A. Music Award for Best New Country Artist in 1997. George also worked as a designer of support equipment for disabled humans and pets and designed the custom wheelchair that she needs to use to avoid straining Lori's back and neck. George's spina bifida means that she is much shorter than her sister and must be raised to her sister's height in order to move around easily. George used a bar stool as the base of the wheelchair.

The Schappell's have maintained their individual personalities. Although they share an apartment, they have separate rooms, and George has pets whereas Lori does not. They have different groups of friends and attend different churches. Lori cuts her hair short and enjoys an occasional drink whereas George has colored her hair and avoids alcohol. Lori speaks in the Pennsylvania Dutch accent of the Reading area, but George has adopted a Southern drawl from her work in country music.

What Are Conjoined Twins?

Conjoined twins are identical twins that fail to separate completely during the mother's pregnancy. Identical twins are also known as monozygotic * because they are formed when a single fertilized egg from the mother divides to form two separate embryos during pregnancy. Conjoined twins are a rare subtype of identical twins that result from incomplete splitting after the twelfth day of embryonic development. The longer the delay in the separation of the two embryos, the more complicated the connections between the conjoined twins are likely to be.

Illustration of conjoined embryos. Conjoined twins are a rare subtype of identical twins that result from incomplete splitting after the twelfth day of embryonic development.

Illustration of conjoined embryos. Conjoined twins are a rare subtype of identical twins that result from incomplete splitting after the twelfth day of embryonic development.
Christian Darkin/Science Source.

The other major category of twins are fraternal * * twins. Dizy-gotic twins are more common than identical or monozygotic twins.

How Common Are Conjoined Twins?

In general, about 1 in every 90 human pregnancies results in twins. About 40 percent of all twins born are fraternal twins in which one baby is male and the other female. The next most common type is female fraternal twins, followed by male fraternal twins, female identical twins, and male identical twins. Conjoined twins are the rarest form of identical twins, occurring once in every 50,000 to 200,000 pregnancies. They are more likely to occur in Southwest Asia and Africa than in Europe or North America, but the reason for this geographical difference is not yet known. Seventy percent of conjoined twins are female.

Conjoined twins are at considerable risk of death during pregnancy; doctors estimate that between 40 and 60 percent of conjoined twins are stillborn and an additional 35 percent only survive one day after birth. Male conjoined twins are more likely than females to die before birth. Of conjoined twins that are born alive, females outnumber males by a ratio of three to one. In the United States, about one live birth in every 200,000 is a set of conjoined twins.

How Are Conjoined Twins Classified?

Conjoined twins are categorized in two major ways. Some doctors distinguish between symmetrical or equal conjoined twins, a condition in which both infants are well developed, and asymmetrical or unequal conjoined twins, in which an incomplete twin is joined to a fully developed sibling. The incomplete twin is sometimes called a parasitic twin because it depends on the overall health and body functions of the complete sibling. About 10 percent of conjoined twins are asymmetrical twins.

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 medical terms for parts of the body. The conjoined twins may share tissues, organ systems, or both, depending on the areas at which their bodies are fused:


Chang and Eng Bunker (1811–1874) were conjoined twins whose country of origin, then called Siam, led to their being called Siamese twins, a term later considered inappropriate for the condition. Chang and Eng were joined at the breastbone by a band of cartilage, but their vital organs functioned independently. They could have been easily separated by modern surgical techniques and equipment, but 19thcentury surgeons did not have the specialized diagnostic techniques, blood banks, and other equipment necessary for such operations. The boys were fortunate that their parents did not abandon them; the midwife who delivered the boys was afraid to touch them for fear that she would be cursed; and when the King of Siam heard about the unusual birth, he condemned the twins to death. Luckily, he never carried out the sentence.

Like other sets of conjoined twins in the 19th century, Chang and Eng were exhibited as a medical curiosity. They were discovered by a British business promoter in 1829 and taken on tour in England and the United States. The twins then joined P. T. Barnum's traveling circus after their contract with the Englishman expired. They left the circus in 1839 and settled in a town in North Carolina. Taking the surname Bunker, they became citizens of the United States and married two sisters in 1843. Chang had 10 children by his wife while Eng had 11. The two wives did not get along, however, and the brothers set up two separate households, spending three days a week in each home.

The twins' health grew worse in the early 1870s, partly as a result of heavy drinking and financial worries following the Civil War. Chang died on January 17, 1874, most likely from a stroke; Eng died an hour later. The twins have been the subject of several novels, plays, and short stories.

How Serious Is the Conjoined Condition?

Between 40 and 80 percent of conjoined twins who need emergency surgery after birth die in intensive care following the operation. In some cases, particularly those involving asymmetrical 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 born alive and are healthy enough to have surgery postponed until they are older have a survival rate of 75 percent for at least one of the twins.

How Are Conjoined Twins Diagnosed and Treated?


In some cases the mother's 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. Ultrasound imaging is also useful in planning for the birth, as many conjoined twins are not in the usual head-down position before birth and the doctor needs to be prepared.

After the twins are born, magnetic resonance imaging * can be used to identify which organ systems are shared between them and which organs are abnormal or incompletely developed. In every set of conjoined twins, one will be smaller than the other and will usually have more abnormal organs as well. CT scans * are primarily useful in evaluating the bony structures of twins fused at the hips or pelvis.

An electrocardiogram * and an electroencephalogram * (EEG) may be used to evaluate the extent of shared heart or brain function, and to determine whether surgical separation of the twins will be possible after birth. Although the number of successful separations of conjoined twins increased after the first attempt in Paris in 1888—about 250 surgical separations had been reported as of 2015—some conjoined twins cannot be separated.


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 are likely to 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 6 to 12 months). Twins who share a heart or brain usually cannot be separated without causing the death of both twins. Emergency surgery is needed when one twin dies shortly after birth in order to save the other twin.

Conjoined twins are usually delivered by cesarean section * rather than waiting for the mother's due date, in part because conjoined twins are more likely than other babies to be born prematurely. The operation is usually scheduled for two to four weeks before the expected date of birth. 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 35 hours to complete. Two complete surgical teams are required to care for the twins during and after the separation. In most cases, conjoined twins who survive separation need further surgery at intervals during childhood as their bodies grow and change.


Complications following the separation of conjoined twins are common in spite of the number of imaging studies and careful planning that takes place before the operation. The younger the infants at the time of surgery, the more likely they are to have complications related to blood loss, changes in blood pressure, or the use of general anesthesia * .

Some complications related to the separation of conjoined twins concern the legal system rather than surgery. In 2000, there was a controversial case in Great Britain in which conjoined girls were separated shortly after birth by order of the High Court over the objections of the twins' parents even though it was certain that the weaker twin would die as a result of the surgery. This case raises questions about a government's right to overrule parents' wishes as well as the ethics * involved in sacrificing one child's life for the sake of the other. In the United States as of 2016, the parents' decision regarding separation was considered final.

Can Conjoined Twins Be Prevented?

As of 2016, conjoined twinning could not be prevented because the cause of the incomplete separation of the two embryos around the twelfth day of pregnancy is not known. Although twin births in general are known to run in families, conjoined twinning is considered to be a random occurrence; the parents of conjoined twins do not have an increased risk of having a second set in a later pregnancy.

Parents can make the difficult choice to terminate the pregnancy if the mother has had an ultrasound or other diagnostic examination and is known to be carrying conjoined twins.

Living as Conjoined Twins

It is possible for conjoined twins who are not separated to have productive and satisfying lives, as the example of the Schappell sisters indicates. Daisy and Violet Hilton (1908–1969) were pygopagus conjoined twins born in England who moved to the United States and worked as vaudeville singers and dancers, including a tap-dancing routine with Bob Hope. A set of conjoined twins born in 1990 in Minnesota obtained two driver's licenses, and graduated from college in 2010. In 2012, they starred in their own reality TV show Abby & Brittany for the TLC Network.

Pediatric neurosurgeon Benjamin Solomon “Ben” Carson, Sr., the first surgeon to successfully separate conjoined twins joined at the head.

Pediatric neurosurgeon Benjamin Solomon “Ben” Carson, Sr., the first surgeon to successfully separate conjoined twins joined at the head.

Alice Dreger, a medical historian from Michigan, wrote in her 2005 book, One of Us: Conjoined Twins and the Future of Normal, that many conjoined twins are satisfied with their lives, and—contrary to popular opinion—do not feel sorry for themselves or see their lives as not worth living. The Schappell sisters, for example, are opposed to separation surgery because they do not want to risk the possibility that one of them might die.

Conjoined twins who are not separated commonly state that being stared at by strangers, regarded as objects of pity, or asked intrusive questions about their bathroom habits or sexual activities are more bothersome than the physical inconveniences of their shared lives. Although a number of pairs of conjoined twins in the 19th and early 20th centuries ended up in circuses or other forms of show business in order to earn a living, changes in employment legislation and the widespread use of computers in schools and workplaces have made it possible for contemporary conjoined twins to go to college and enter other occupations.

Conjoined twins who are not separated, however, have shortened life expectancies; most pairs die in their 20s or early 30s. The oldest known living set of conjoined twins is a pair of brothers in Ohio born in 1951.


Books and Articles

Kobylarz, K. “Intensive Care of Conjoined Twins.” Anesthesiology Intensive Therapy 46 no. 2 (April-June 2014): 130–136.

Kumar, Bid, and Alfirevic, Zarko, eds. Fetal Medicine. Cambridge, UK: Cambridge University Press, 2016.


Mayo Clinic. “Conjoined Twins.” (accessed March 6, 2016).

The University of Maryland Medical Center. “Facts About Conjoined Twins.” (accessed March 6, 2016).


American Congress of Obstetricians and Gynecologists. 409 12th Street SW, Washington, DC 20024. Telephone: 202-638-5577. Toll-free: 800-673-8444. Website: (accessed March 6, 2016).

Multiples of America. 2000 Mallory Lane, Suite 130-600, Franklin, TN 37067. Telephone: 248-231-4480. Website: (accessed March 6, 2016).

* identical twins are twins produced when a single egg from the mother is fertilized and divides to form two separate embryos of the same sex with nearly identical DNA.

* embryo (EM-bree-o), in humans, is the developing organism from the end of the second week after fertilization to the end of the eighth week.

* spina bifida (SPY-nuh BIF-ih-duh) is a condition present at birth in which the spinal column is imperfectly closed, leaving part of the spinal cord exposed and often leading to neurological and other problems.

* monozygotic (mah-no-zye-GOTik) means derived from a single fertilized egg.

* fraternal twins are born at the same time but develop from two separate fertilized eggs. Unlike identical twins, who develop from only one fertilized egg that splits into two and who look exactly alike, fraternal twins may not look the same at all or be the same gender. Identical twins have the same genes, but fraternal twins are no more likely to share genes than non-twin siblings.

* dizygotic (dye-zye-GOT-ik) derived from two different fertilized eggs.

* ultrasound also called a sonogram, is a diagnostic test in which sound waves passing through the body create images on a computer screen.

* magnetic resonance imaging or MRI uses magnetic waves, instead of x-rays, to scan the body and produce detailed pictures of the body's structures.

* CT scans, computed tomography (to-MOG-ra-fee) scans, or CAT (computerized axial tomography) scans, use computers to view structures inside the body.

* electrocardiogram (e-lek-troKAR-dee-o-gram), also known as an EKG, is a test that records and displays the electrical activity of the heart.

* electroencephalogram is an instrument that records the electrical activity of the brain.

* cesarean section (si-ZAR-ee-an SEK-shun) is the surgical incision of the walls of the abdomen and uterus to deliver offspring in cases where the mother cannot deliver through the vagina.

* general anesthesia (an-es-THE-zha) means using drugs or inhaled gases to create a state of unconsciousness and muscle relaxation throughout the body to block pain during surgery. Local anesthesia blocks or numbs pain in one part of the body while patients remain awake.

* ethics is a guiding set of principles for conduct, a system of moral values.

Disclaimer:   This information is not a tool for self-diagnosis or a substitute for professional care.

(MLA 8th Edition)