Conjoined twins are identical twins joined in utero. A rare phenomenon, the occurrence is estimated to range from 1 in 49,000 births to 1 in 189,000 births, with a somewhat higher incidence in Southwest Asia, Africa and Brazil. Approximately half are stillborn, and a smaller fraction of pairs born alive have abnormalities incompatible with life. The condition is more frequently found among females, with a ratio of 3:1.
Two contradicting theories exist to explain the origins of conjoined twins. The older theory is fission, in which the fertilized egg splits partially. The second and more generally accepted theory is fusion, in which a fertilized egg completely separates, but stem cells (which search for similar cells) find like-stem cells on the other twin and fuse the twins together. Conjoined twins share a single common chorion, placenta, and amniotic sac, although these characteristics are not exclusive to conjoined twins as there are some monozygotic but non-conjoined twins that also share these structures in utero.
The most famous pair of conjoined twins was Chang and Eng Bunker (Thai: อิน-จัน, In-Chan) (1815–1880), Thai brothers born in Siam, now Thailand. They travelled with P.T. Barnum's circus for many years and were labeled as the Siamese Twins. Chang and Eng were joined by a band of flesh, cartilage, and their fused livers at the torso. In modern times, they could have been easily separated. Due to the brothers' fame and the rarity of the condition, the term "Siamese twins" came to be used as a synonym for conjoined twins.
Conjoined twins are typically classified by the point at which their bodies are joined. The most common types of conjoined twins are:
Thoraco-omphalopagus (28% of cases): Two bodies fused from the upper chest to the lower chest. These twins usually share a heart, and may also share the liver or part of the digestive system.
Thoracopagus (18.5%): Two bodies fused from the upper thorax to lower belly. The heart is always involved in these cases.
Omphalopagus (10%): Two bodies fused at the lower abdomen. Unlike thoracopagus, the heart is never involved in these cases; however, the twins often share a liver, digestive system, diaphragm and other organs.
Parasitic twins (10%): Twins that are asymmetrically conjoined, resulting in one twin that is small, less formed, and dependent on the larger twin for survival.
Craniopagus (6%): Fused skulls, but separate bodies. These twins can be conjoined at the back of the head, the front of the head, or the side of the head, but not on the face or the base of the skull.
Other less-common types of conjoined twins include:
Cephalopagus: Two faces on opposite sides of a single, conjoined head; the upper portion of the body is fused while the bottom portions are separate. These twins generally cannot survive due to severe malformations of the brain. Also known as janiceps (after the two-faced god Janus) or syncephalus.
Syncephalus: One head with a single face but four ears, and two bodies.
Cephalothoracopagus: Bodies fused in the head and thorax. In this type of twins, there are two faces facing in opposite directions, or sometimes a single face and an enlarged skull.
Xiphopagus: Two bodies fused in the xiphoid cartilage, which is approximately from the navel to the lower breastbone. These twins almost never share any vital organs, with the exception of the liver. A famous example is Chang and Eng Bunker.
Ischiopagus: Fused lower half of the two bodies, with spines conjoined end-to-end at a 180° angle. These twins have four arms; two, three or four legs; and typically one external set of genitalia and anus.
Omphalo-Ischiopagus: Fused in a similar fashion as ischiopagus twins, but facing each other with a joined abdomen akin to omphalopagus. These twins have four arms, and two, three, or four legs.
Parapagus: Fused side-by-side with a shared pelvis. Twins that are dithoracic parapagus are fused at the abdomen and pelvis, but not the thorax. Twins that are diprosopic parapagus have one trunk and two faces. Twins that are dicephalic parapagus are dicephalic, and have two (dibrachius), three (tribrachius), or four (tetrabrachius) arms.
Pygopagus (Iliopagus): Two bodies joined at the pelvis.
Rachipagus: Twins joined along the dorsal aspect (back) of their bodies, with fusion of the vertebral arches and the soft tissue from the head to the buttocks
Surgery to separate conjoined twins may range from very easy to very hard, depending on the point of attachment and the internal parts that are shared. Most cases of separation are extremely risky and life-threatening. In many cases, the surgery results in the death of one or both of the twins, particularly if they are joined at the head or share a vital organ. This makes the ethics of surgical separation, where the twins can survive if not separated, contentious. Alice Dreger of Northwestern University found the quality of life of twins who remain conjoined to be higher than is commonly supposed.Lori and George Schappell are a good example.
In 1957, Bertram Katz and his surgical team made international medical history performing the world's first successful separation of conjoined twins sharing a vital organ. Omphalopagus twins John Nelson and James Edward Freeman (Johnny and Jimmy) were born to Mr. and Mrs. William Freeman of Youngstown, Ohio, on April 27, 1956. The boys shared a liver but had separate hearts and were successfully separated at North Side Hospital in Youngstown, Ohio by Bertram Katz. The operation was funded by the Ohio Crippled Children's Service Society.
In 1987, Ben Carson made medical history by being the first surgeon to successfully separate conjoined twins (the Binder twins) conjoined at the back of the head (craniopagus twins). Operations to separate twins joined in this way had always failed, resulting in the death of one or both of the infants. Carson agreed to undertake the operation. The 50-member surgical team, led by Carson, worked for 22 hours. At the end, the twins were successfully separated and can now survive independently.
As Carson said in an interview:
I was talking to a friend of mine, who was a cardiothoracic surgeon, who was the chief of the division, and I said, "You guys operate on the heart in babies, how do you keep them from exsanguinating" and he says, "Well, we put them in hypothermic arrest." I said, "Is there any reason that -- if we were doing a set of Siamese twins that were joined at the head -- that we couldn't put them into hypothermic arrest, at the appropriate time, when we're likely to lose a lot of blood?" and he said, "No." I said, "Wow, this is great." Then I said, "Why am I putting my time into this? I'm not going to see any Siamese twins." So I kind of forgot about it, and lo and behold, two months later, along came these doctors from Germany, presenting this case of Siamese twins. And, I was asked for my opinion, and I then began to explain the techniques that should be used, and how we would incorporate hypothermic arrest, and everybody said "Wow! That sounds like it might work." And, my colleagues and I, a few of us went over to Germany. We looked at the twins. We actually put in scalp expanders, and five months later we brought them over and did the operation, and lo and behold, it worked.
Recent successful separations of conjoined twins include that of the separation of Ganga & Jamuna Shreshta in 2001, who were born in Kathmandu, Nepal in 2000. The 197-hour surgery on the pair of craniopagus twins was a landmark one which took place in Singapore; the team was led by neurosurgeons, Chumpon Chan and Keith Goh. The surgery left Ganga with brain damage and Jamuna unable to walk. Seven years later, Ganga Shrestha died at the Model Hospital in Katmandu in July 2009, at the age of 8, three days after being admitted for treatment of a severe chest infection.
A case of particular interest was that of infants Rose and Grace ("Mary" and "Jodie") Attard, conjoined twins from Malta who were separated in Great Britain by court order Re A (Children) (Conjoined Twins: Surgical Separation) over the religious objections of their parents, Michaelangelo and Rina Attard. The surgery took place in November, 2000, at St Mary's Hospital in Manchester. The operation was controversial because Rose, the weaker twin, would die as a result of the procedure as her heart and lungs were dependent upon Grace's. (The twins were attached at the lower abdomen and spine.) However, if the operation had not taken place, it was certain that both twins would die. Grace survived to enjoy a normal childhood.
In 2001 two 36-year-old women from Iran, Ladan and Laleh Bijani, who were joined at the head but had separate brains (craniopagus) were surgically separated in Singapore, despite surgeons' warnings that the operation could be fatal to one or both. Their complex case was accepted only because high tech graphical imagery and modelling would allow the medical team to plan the risky surgery. Unfortunately, an undetected major vein hidden from the scans was discovered during the operation. The separation was completed but both women died while still in surgery on July 8, 2003.
The Moche culture of ancient Peru depicted conjoined twins in their ceramics dating back to 300 CE. Writing around 415CE, St. Augustine of Hippo in his book City of God refers to a man "double in his upper, but single in his lower half--having two heads, two chests, four hands, but one body and two feet like an ordinary man." The earliest known documented case of conjoined twins dates from the year 942, when a pair of conjoined twin brothers from Armenia were brought to Constantinople for medical evaluation.
In Arabia, the twin brothers Hashim ibn Abd Manaf and 'Abd Shams were born with Hashim's leg attached to his twin brother's head. Legend says that their father, Abd Manaf ibn Qusai, separated his conjoined sons with a sword and that some priests believed that the blood that had flowed between them signified wars between their progeny (confrontations did occur between Banu al'Abbas and Banu Ummaya ibn 'Abd Shams in the year 750 AH). The MuslimpolymathAbū al-Rayhān al-Bīrūnī described Siamese twins in his book Kitab-al-Saidana.
The English twin sisters Mary and Eliza Chulkhurst, who were conjoined at the back (pygopagus), lived from 1100 to 1134 (or 1500 to 1534) and were perhaps the best-known early historical example of conjoined twins. Other early conjoined twins to attain notice were the "Scottish brothers", allegedly of the dicephalus type, essentially two heads sharing the same body (1460–1488, although the dates vary); the pygopagus Helen and Judith of Szőny, Hungary (1701–1723), who enjoyed a brief career in music before being sent to live in a convent; and Rita and Cristina of Parodi of Sardinia, born in 1829. Rita and Cristina were dicephalus tetrabrachius (one body with four arms) twins and although they died at only eight months of age, they gained much attention as a curiosity when their parents exhibited them in Paris.
Several sets of conjoined twins lived during the nineteenth century and made careers for themselves in the performing arts, though none achieved quite the same level of fame and fortune as Chang and Eng. Most notably, Millie and Christine McCoy (or McKoy), pygopagus twins, were born into slavery in North Carolina in 1851. They were sold to a showman, J.P. Smith, at birth, but were soon kidnapped by a rival showman. The kidnapper fled to England but was thwarted because England had already banned slavery. Smith traveled to England to collect the girls and brought with him their mother, Monimia, from whom they had been separated. He and his wife provided the twins with an education and taught them to speak five languages, play music, and sing. For the rest of the century the twins enjoyed a successful career as "The Two-Headed Nightingale" and appeared with the Barnum Circus. In 1912 they died of tuberculosis, 17 hours apart.
Giovanni and Giacomo Tocci, from Locana, Italy, were immortalized in Mark Twain's short story "Those Extraordinary Twins" as fictitious twins Angelo and Luigi. The Toccis, born in 1877, were dicephalus tetrabrachius twins, having one body with two legs, two heads, and four arms. From birth they were forced by their parents to perform and never learned to walk, as each twin controlled one leg (in modern times physical therapy allows twins like the Toccis to learn to walk on their own). They are said to have disliked show business. In 1886, after touring the United States, the twins returned to Europe with their family, where they fell very ill. They are believed to have died around this time, though some sources claim they survived until 1940, living in seclusion in Italy.
List of notable conjoined twins
Names listed in boldface are of twins who have been separated.
Born 19th century and earlier
Mary and Eliza Chulkhurst (born 1100, also known as the Biddenden Maids) from England. They are the earliest set of conjoined twins whose names are known.
Zita and Gita Rezahanova (born October 19, 1991 in Kyrgyzstan) also had form of dicephalus tetrabrachius tripus. They were successfully separated in 2003 in Moscow, at the current moment they are graduating from high school.
Ganga and Jamuna Mondal, known professionally as The Spider Girls and The Spider Sisters.
Lakshmi Tatma (born 2005) was an ischiopagus conjoined twin born in Araria district in the state of Bihar, India. She had four arms and four legs, resulting from a joining at the pelvis with a headless undeveloped parasitic twin.
Emma and Taylor Bailey, thoracopagus twins born in Arizona, USA in 2006. Joined at the chest and sharing a 7-chamber heart and liver. Both died during a preparatory heart surgery August 10, 2010 at age 3 in Seattle, USA.
Rital and Ritaj Gaboura, had been born in Khartoum, Sudan in 2010 before they were separated in London in 2011. They were craniopagus joined by tops of their heads.
Unnamed boy twins, born in Nakuru, Kenya in May 2013. Sharing most organs, including heart, liver, two legs, 3 visible arms, and one set of sexual organs. Separate heads. Died after 5 days.
Unnamed boy Dicephalic Parapagus twins, born in Jaipur, Rajasthan, India in July 2013 . The boys' heads, nervous systems and backbones are separate. The backbones are joined below the pelvis and they share a rib cage and shoulder girdle. Died after six days.
Emmett and Owen Ezell, thoracic-omphalopagus twins, were born on July 15, 2013 and successfully separated on August 24, 2013.
Ousainatou and Hasanatou Jallow, thoracopagus twins born in Serrekunda in Gambia on the 6th of August 2008 to one Ramatoulie Jallow. They lived for eight days.
Tinashe and Tinotenda Mufuka, omphalopagus twins born in Chiweshe in Zimbabwe in July 2003. Dr. Jacob Langer of SickKids Hospital successfully separated them at about nine months of age. 
Kupakwashe and Tapiwanashe Chitiyo, omphalopagus twins born in Murehwa in Zimbabwe in April 2014. They were successfully separated at two months of age by a team lead by Dr. Bothwell Mbuwayesango.