Conjoined/Siamese twins are identical twins whose bodies are joined in utero. A rare phenomenon, the occurrence is estimated to range from 1 in 50,000 births to 1 in 200,000 births, with a somewhat higher incidence in Southwest Asia and Africa. Approximately half are stillborn, and a smaller fraction of pairs born alive have abnormalities incompatible with life. The overall survival rate for conjoined twins is approximately 25%. 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 and most generally accepted theory is fission, in which the fertilized egg splits partially. The second 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.
The most famous pair of conjoined twins was Chang and Eng Bunker (Thai: อิน-จัน, In-Chan) (1811–1874), Thai brothers born in Siam, now Thailand They traveled with P.T. Barnum's circus for many years and were billed 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 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 chest. 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.
- Synecephalus: 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 one head with two faces. Twins that are dicephalic parapagus have one trunk and two heads, and two (dibrachius), three (tribrachius), or four (tetrabrachius) arms.
- Craniopagus parasiticus: Like craniopagus, but with a second bodiless head attached to the dominant head.
- Pygopagus (Iliopagus): Two bodies joined back-to-back at the buttocks.
- urgery to separate conjoined twins may range from relatively simple to extremely complex, 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. This makes the ethics of surgical separation, where the twins can survive if not separated, contentious. Dreger 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.
- case of particular interest was that of Mary and Jodie, two conjoined twins from Malta who were separated by court order in Great Britain 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 it was certain that the weaker twin, Mary, would die as a result of the procedure. (The twins were attached at the lower abdomen and spine; Jodie's heart and lungs supplied both of their bodies.) However, if the operation had not taken place, it was certain that both twins would die.
- he Moche culture of ancient Peru depicted conjoined twins in their ceramics dating back to 300 CE. The earliest known documented case of conjoined twins dates from the year 945, when a pair of conjoined twin brothers from Armenia were brought to Constantinople for medical evaluation. It was here that they were determined to be acts of God and the birth of conjoined twins was considered a proof that the male's sexual prowess was truly twice that of the average man.
- he English twin sisters Mary and Eliza Chulkhurst, who were conjoined at the back (pygopagus), lived from 1100 to 1134 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.
- everal 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.
- iovanni 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.
- he life of Abd Manaf ibn Qusai includes a legend that he separated his conjoined sons with a sword.
- Mary and Eliza Chulkhurst (1100-1134) (also known as the Biddenden Maids) from England. They are the earliest set of conjoined twins whose names are known.
- Lazarus and Joannes Baptista Colloredo (1617-164?)
- Helen and Judith of Szony (Hungary, 1701–1723)
- Chang and Eng Bunker (1811-1874), from Thailand (formerly Siam), joined by the areas around their xiphoid cartilages, but over time the join stretched; the expression Siamese twins is derived from their case
- Millie and Christine McCoy (July 11, 1851 - October 8, 1912) were American conjoined twins who went by the stage names "The Two-Headed Nightingale" and "The Eighth Wonder of the World".
- Giacomo and Giovanni Battista Tocci (1875?-1912?)
- Rosa and Josepha Blazek of Bohemia (modern-day Czech Republic (1878–1922)). Rosalie gave birth in 1910 to a son, in the only recorded instance of a conjoined twin becoming pregnant and carrying a child to term.
- Radica and Doodica were born in Orissa, India in 1888. They were xiphopagus twins, joined at the chest by a band of cartilage, similar to Chang and Eng. The sisters were separated in Paris by Dr. Eugène-Louis Doyen with the hope of saving Radica. Dr. Doyen was a pioneering medical filmmaker and filmed the twins' surgery as La Separation de Doodica-Radica. Though the operation was considered a success at first, Doodica died shortly after separation, and Radica also succumbed to tuberculosis in 1903, having lived the last year of her life in a Paris sanitorium.