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Conjoined twins, also known as Siamese twins, are identical twins joined in utero. A very 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 and Africa. Approximately half are stillborn, and an additional one-third die within 24 hours. Most live births are female, with a ratio of 3:1.
Two contradicting theories exist to explain the origins of conjoined twins. The more generally accepted theory is fission, in which the fertilized egg splits partially. The other theory, no longer believed to be the basis of conjoined twinning, is fusion, in which a fertilized egg completely separates, but stem cells (which search for similar cells) find similar 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 who also share these structures in utero.
Chang and Eng Bunker (1811–1874) were brothers born in Siam who traveled widely for many years and were labeled as The Siamese Twins. Chang and Eng were joined at the torso by a band of flesh, cartilage, and their fused livers. 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.
There are two theories about the development of conjoined twins. The first is that a single fertilized egg does not fully split during the process of forming identical twins. If the zygote division occurs after 2 weeks of the development of the embryonic disc, it results in the formation of conjoined twins. The second theory is that a fusion of two fertilized eggs occurs earlier in development.
Partial splitting of the primitive node and streak may result in formation of conjoined twins. These twins are classified according to the nature and degree of their union. Occasionally, monozygotic twins are connected only by a common skin bridge or by a common liver bridge. The type of twins formed depends on when and to what extent abnormalities of the node and streak occurred. Misexpression of genes, such as Goosecoid, may also result in conjoined twins. Goosecoid activates inhibitors of BMP4 and contributes to regulation of head development. Over or underexpression of this gene in laboratory animals results in severe malformations of the head region, including duplications, similar to some types of 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 chest to lower belly. The heart is always involved in these cases. As of 2015[update], separation of a genuinely shared heart has not offered survival to two twins; a designated twin may survive if allotted the heart, sacrificing the other twin.
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 Roman deity 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; one, two, three or four legs; and typically one external set of genitalia and anus.
Omphalo-Ischiopagus: Fused in a similar fashion to 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 have one trunk and two heads, and have two (dibrachius), three (tribrachius), or four (tetrabrachius) arms.
Pygopagus or Iliopagus: Two bodies joined at the pelvis.
Rachipagus: Twins joined along the 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 difficult 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 and Abby and Brittany Hensel are notable examples.
The first record of separating conjoined twins took place in the Byzantine Empire in the 900s. One of the conjoined twins had already died, so the doctors of the town attempted to separate the dead twin from the surviving twin. The result was partly successful as the remaining twin lived for three days after separation. The next case of separating conjoined twins was recorded in 1689 in Germany several centuries later. The first recorded successful separation of conjoined twins was performed in 1689 by Johannes Fatio. In 1955, neurosurgeon Harold Voris (1902-1980) and his team at Mercy Hospital in Chicago performed the first successful operation to separate craniopagus twins (conjoined at the head), which resulted in long-term survival for both. The larger girl was reported in 1963 as developing normally, but the smaller was permanently impaired.
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 in 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.
Recent successful separations of conjoined twins include that of the separation of Ganga and Jamuna Shreshta in 2001, who were born in Kathmandu, Nepal, in 2000. The 97-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 Kathmandu in July 2009, at the age of eight, three days after being admitted for treatment of a severe chest infection.
Infants Rose and Grace Attard, conjoined twins from Malta, were separated in Great Britain by court order Re A over the religious objections of their parents, Michaelangelo and Rina Attard. The twins were attached at the lower abdomen and spine. 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. However, if the operation had not taken place, it was certain that both twins would die. Grace survived to enjoy a normal childhood.
In 2003, two 29-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 technologically advanced graphical imagery and modelling would allow the medical team to plan the risky surgery. However, 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.
In 2019 Safa and Marwa Ullah were separated at Great Ormond Street Hospital in London, England. The twins, born January 2017 were joined at the top of the head with separate brains and a cylindrical shared skull with the twins each facing in opposite directions to one another. The surgery was jointly led by neurosurgeon Owase Jeelani and plastic surgeon Professor David Dunaway. The surgery presented particular difficulties due to a number of shared veins and a distortion in the shape of the girls brains, causing them to overlap. The distortion would need to be corrected in order for the separation to go ahead. The surgery utilized a team of more than 100 including bio engineers, 3D modelers and a virtual reality designer. The separation was completed in February 2019 following a total of 52 hours of surgery over three separate operations. As of July 2019 both girls remain healthy and the family plans to return to their home in Pakistan in 2020.
The Moche culture of ancient Peru depicted conjoined twins in their ceramics dating back to 300 CE. Writing around 415 CE, 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."
According to Theophanes the Confessor, a Byzantine historian of the 9th century, around 385/386 CE, "in the village of Emmaus in Palestine, a child was born perfectly normal below the navel but divided above it, so that it had two chests and two heads, each possessing the senses. One would eat and drink but the other did not eat; one would sleep but the other stayed awake. There were times when they played with each other, when both cried and hit each other. They lived for a little over two years. One died while the other lived for another four days and it, too, died."
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 MuslimpolymathAb? al-Rayh?n al-B?r?n? described conjoined 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 Sz?ny, 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 ill. They are believed to have died around this time, though some sources claim they survived until 1940, living in seclusion in Italy.
Born 19th century and earlier
Chang and Eng Bunker, watercolor on ivory, 1835 or 1836
Mary and Eliza Chulkhurst, alleged names of the Biddenden Maids (per tradition, born in the 12th century) of Kent, England. They are the earliest set of conjoined twins whose names are (purportedly) known.
Anna and Barbara Rozycki (born 1970), the first conjoined twins successfully separated.
Ma Nan Soe and Ma Nan San (born 1971 in Myanmar), separated in July 1971 at Yangon Pediatric Hospital. They were joined from chest to belly button. Ma Nan San died after one month and seven days after operation.
Elisa and Lisa Hansen, Ogden, Utah. Born by Caesarean section on 18 October 1977, were conjoined at the top of their head (craniopagus). They were separated 1979 after 16 hour surgery, were first to both survive surgery.
Baby Girl A and Baby Girl B (born 1977 in New Jersey) shared a single six-chambered heart. Separation surgery, led by C. Everett Koop, involved the instant death of Baby Girl A; the difficult ethical and religious concerns generated significant local newspaper coverage. Baby Girl B survived for 6 months.
Tabea and Lea Block, from Lemgo, Germany, were born as craniopagus twins joined on the tops of their heads on August 9, 2003. The girls shared some major veins, but their brains were separate. They were separated on September 16, 2004, although Tabea died about 90 minutes later.
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.
On 2005 a set of conjoined triplets was detected, characterized as tricephalus, tetrabrachius, and tetrapus parapagothoracopagus, and the pregnancy interrupted at 22 weeks.
Aung Myat Kyaw and Aung Khant Kyaw (born in May 2011, Mandalay, Myanmar) connected at pelvis.
Jesus and Emanuel de Nazaré are dicephalic parapagus twins born in Pará, Brazil on December 19, 2011.
Zheng Han Wei and Zheng Han Jing, born in China on August 11, 2013. Conjoined by their sternum, pericardium, and liver. In 2014, they were separated in Shanghai, China, at the Shanghai Children's Medical Center.
Asa and Eli Hamby were born in 2014 in Georgia but died less than two days after birth due to heart failure. The twins were dicephalic parapagus having two heads but being conjoined at the torso, arms and legs. They had separate spinal columns but one heart making postnatal operations impossible.
Erin and Abby Delaney, born in Philadelphia, Pennsylvania USA on July 24, 2016. Conjoined by the head. They were successfully separated at Children's Hospital of Philadelphia on June 16, 2017.
Marieme and Ndeye Ndiaye, twin girls born in Senegal in 2017, living in Cardiff, UK in 2019
Safa and Mara Bibi, twin girls born in Hayatabad, Pakistan on January 17, 2017, coinjoined by the head. Successfully separated at Great Ormond Street Hospital in February 2019.
Callie and Carter Torres, born 30 January 2017 in Houston Texas, from Blackfoot Idaho. They are Omphalo-Ischiopgagus conjoined twins, attached by their pelvic area and sharing all organs from the belly button down with just one leg each.
Yi?it and Derman Evrensel, twin boys born on 21 June 2018, Antalya, Turkey. They are Craniophagus twins and were separated at Great Ormond Street Hospital in 2019 by the same surgeons that separated Safa and Mara Bibi.
In the TV series The Addams Family, there are extended family members of the Addams Family who are mentioned to have two heads. In "Mother Lurch Visits the Addams Family," Morticia Addams mentions that she has a Cousin Slimy who has two good heads on his shoulder. In "Progress and the Addams Family," Morticia was making a knitted hat for Cousin Plato where Gomez Addams has stated that his left head is size 6 and his right head is size 8 3/4. In "Lurch's Little Helper," Morticia made a portrait of Cousin Crimp who has a male head and a female head.
In Lilo & Stitch: The Series, Swapper are Ischiopagus twin green stubby limbed lizard-like experiment with black eyes, purple markings on his back and three purple-tipped tendrils on each head that can emit a green ray from each head's eyes once each head is pointed at one of two individuals, but one time he hit four beings at once, although the complete limit of beams is unknown. The ray will swap the minds and voices of the targets, as well as vice versa, and the only means of returning to normal is through Swapper choosing to do so. Because Swapper is two heads on the same body, Swapper is two beings cooperating as one, though their personalities mirror each other: they can be indecisive at times but usually work well together. An example is when Stitch in Lilo's body asked the two-headed cousin to switch their minds back, Swapper, after hesitating, blew a raspberry in response. The two enjoy messing with people by swapping their brains and refuse to switch them back unless threatened. However, Swapper will eventually switch individuals' minds back into their original bodies when both heads choose to do so.
In Big-Top Pee-wee, the Cabrini Circus has some conjoined twins named Ruth and Dot (portrayed by Helen Infield Siff and Carol Infield Sender).
In The Addams Family and Addams Family Values, there are conjoined twins named Flora and Fauna Amor (portrayed by Darlene and Maureen Sue Levin) who were once dates to Gomez Addams and Uncle Fester. Both films also featured a two-headed relative named Dexter and Donald Addams (portrayed by Douglas Brian Martin and Steven M. Martin).
In The Addams Family cartoon in 1992, the episode "N.J. Addams" featured Aunt Noggin who was a two-headed person who wears a Victorian dress. One head is black and speaks in a Jamaican accent and the other head is Caucasian and speaks in a Brooklyn accent.
The Oblongs, depicts Biff Oblong (Randy Sklar) and Chip Oblong (Jason Sklar)—17-year-old conjoined twins who are attached at the waist and share a middle leg due to their valley's pollution and radiation.
In the DC Comics series Hitman, villain Moe and Joe Dubelz is a conjoined twin gangster. Moe was alive at the time of introduction, but Joe had already died and is, in fact, undergoing putrefaction.
In the episode "Humbug" of The X-Files, Vincent Schiavelli portrayed a circus performer named Lanny, with an underdeveloped conjoined twin named Leonard. The episode also includes a reference to Chang and Eng.
The Prophet of Yonwood has a reference to Chang and Eng when the main character, Nickie, finds a picture of them in her great-grandfather's old house in Siam.
In the anime Naruto, Sakon () and his conjoined twin brother Ukon () are the strongest of the Sound Four and count as one member due to their abilities to merge bodies and kill an opponent at a cellular level. They both serve as antagonists.
The Broadway musical Side Show depicts the lives of real-life conjoined twins Daisy and Violet Hilton, portrayed in the original Broadway production by Alice Ripley and Emily Skinner.
The American medical drama Grey's Anatomy featured several cases of conjoined twins.
The musical group Evelyn, Evelyn depicts a pair of conjoined twin sisters—often referred to as "The Evelyn Sisters"—in many of their songs and music videos. The fictional sisters are shown to be child prostitutes in the music video for "Sandy Fishnets", and the song "Evelyn, Evelyn" describes their longing for privacy and to be separated from one another.
The animated series Duckman featured Eric T. Duckman's sons Charles (voiced by Dana Hill in 1994-1996, Pat Musick in 1997) and Mambo (voiced by E. G. Daily) who are dicephalic parapagus twins where their heads share a body.
Fran Bow, a 2015 indie psychological horror game, includes Clara and Mia Buhalmet, a set of mentally ill conjoined twins, as characters. They were surgically sewn together, much like an experiment performed by Josef Mengele, also known as the Angel of Death, in which a pair of twins were sewn together back to back by blood vessels and organs, in an attempt to create conjoined twins.
The Peach Tree, a Korean novel and film, portrays conjoined twin brothers falling in love with the same woman.
The 1999 movie Twin Falls Idaho portrays conjoined twin brothers who are played by two non-conjoined identical twin brothers, one of whom directed the film, and both of whom co-wrote the screenplay.
On the television series Ruby Gloom, the characters Frank and Len are conjoined twins who comprise a rock group called RIP.
In the film Monsters University, two of the members of fictitious fraternity Oozma Kappa are named Terri and Terry Perry (voiced by Sean Hayes and Dave Foley). They are dicephalic parapagus twins where they have four arms and share the same tentacles that are in place of their legs.
^The Chronicle of Theophanes Confessor. Byzantine and Near Eastern History, AD 284-813. Translated with Introduction and Commentary by Cyril Mango and Roger Scott. Oxford: Clarendon Press, 1997, p. 106-107.
^The Life of the Prophet Muhammad: Al-Sira Al-Nabawiyya By Ibn Kathir, Trevor Le Gassick, Muneer Fareed, pg. 132
^Bondeson, Jan (April 1992), "The Biddenden Maids: a curious chapter in the history of conjoined twins", Journal of the Royal Society of Medicine, London: Royal Society of Medicine Press, 85 (4): 217-221, PMC1294728, PMID1433064
^Staffenberg, David A.; Goodrich, James T. (January 2005). "Separation of craniopagus conjoined twins: an evolution in thought". Clinics in Plastic Surgery. 32 (1): 25-34. doi:10.1016/j.cps.2004.09.002. PMID15636762.
^Athanasiadis, Apostolos P.; Tzannatos, Christinne; Mikos, Themistoklis; Zafrakas, Menelaos; Bontis, John N. (June 2005). "A unique case of conjoined triplets". American Journal of Obstetrics and Gynecology. 192 (6): 2084-2087. doi:10.1016/j.ajog.2004.10.622. PMID15970906.