Conjoined twins are twins whose bodies are joined together at birth. This happens where the zygote of identical twins fails to completely separate. Conjoined twins occur in an estimated one in 200,000 births, with approximately half being stilborn. The overall survival rate for conjoined twins is between 5% and 25%. Conjoined twins are more likely to be female (70-75%).

Perhaps the most famous pair of conjoined twins were Chang and Eng Bunker (1811 - 1874), Chinese brothers born in Siam, now Thailand. They traveled with a circus for many years and were billed as the Siamese Twins; due to their notoriety and the rarity of the condition, today the term is frequently used as a synonym for conjoined twins. Chang and Eng were joined by a band of flesh, cartilage,and their shared liver at the torso. In modern times, they could have been separated easily.

Conjoined twins form in two ways. The first is fission, in which the fertilized egg splits partially, resulting in two-to-one type conjoined twins, i.e. Dicephalus twins. The second is fusion, a more common type of conjoined twinning, is when a fertilized egg completely seperates, but stem cells (which search for similar cells) find like-stem cells on the other twin and fuses the twins together. This results in single and relative organ sharing, i.e. Thorapagus twins.

There are several different types of conjoined twins, which are......

-- Diplopagus: Conjoined twins joined equally with near complete body, only sharing a few organs.

-- Heteropagus: Conjoined twins joined unequal usually resulting in parasitic twin.

• Thoracopagus: Bodies fused in the thorax. The heart is always involved in these cases; when the heart is shared, prospects for a long life, either with or without separation surgery, are poor (35-40% of cases).
• Omphalopagus: Joined at the lower chest. The heart is not involved in these cases but the twins often share a liver, digestive system, diaphragm and other organs (34% of cases).
  • Xiphopagous: bodies fused in the xiphoid cartilage, e.g., Chang and Eng
• Pygopagus (iliopagus): Joined, usually back to back, to the buttocks (19% of conjoined twins).
• Cephalopagus: Heads fused, bodies separated. These twins generally cannot survive due to severe malformations of the brain. Also known as janiceps (after the two-faced god Janus) or syncephalus.
  • Cephalothoracopagus: Bodies fused in the head and thorax. These twins also generally cannot survive. (Also known as epholothoracopagus or craniothoracopagus.)
• Craniopagus: Skulls fused, but bodies separate (2%).
  • Craniopagus parasiticus - A second bodiless head attached to the head.
• Dicephalus: Two heads, one body with two legs and two, three, or four arms (dibrachius, tribrachius or tetrabrachius, respectively.) Abigail and Brittany Hensel, 17-year-old conjoined twins from the United States, are of the dicephalus tribrachius type, with their third arm having been removed while they were very young.
• Ischiopagus: Anterior union of the lower half of the body, with spines conjoined at a 180° angle (6% of cases). Or with the spines separate but both the pelvises forming a single big ring which includes two sacrums and two public symphyses.
• Ischio-omphalopagus: The most well known type of conjoined twins. The Twins are conjoined with spines in a Y-shape. They have four arms and usually two or three legs. These cases can be challenging because the twins often share reproductive and excretory systems.
• Parapagus: lateral union of the lower half extending variable distances upward, with the heart sometimes involved (5% of cases).
• Diprosupus: One head, with two faces side by side.

In some cases, parts of the brain have been known to be shared between conjoined twins joined at the head.

Occasionally one of the twins will fail to develop properly, effectively acting as a parasite upon the normally developed twin: this condition is known as parasitic twinning or heteropagus twins. One twin may absorb the other, which is known as inclusion twinning.......