The etymology of term "siamese twins" can be attributed to Chang and Eng Bunker, the famous conjoined twins from Siam who earned their living in the U.S. as a circus attraction in the Barnum and Baily Circus. They were born on May 11, 1811 in Siam and eventually made their way to the United States where they earned a living first as circus attractions and then as farmers. Scientists of the day argued over whether the brothers could be successfully separated, since the twins joined at the chest by a thick band of tissue. Twins joined this way can now be separated, but Chang and Eng did not have 20th century technological advances at their disposal. Nevertheless, they lived long and fruitful lives until their death at age 63. Both married and fathered 21 children between them. Interestingly, 2 of their grand-daughters produced 2 normal sets of twins. However, the story of Chang and Eng is an exceptional one, since so few conjoined twins survive beyond infancy, let alone are born in the first place.
Siamese twins are the result of a rare embryological accident. The developing embryo begins to split into identical twins but then stops part way leaving the partially separated egg to mature into a fetus. Most conjoined twins are stillborn, and those that survive often die within a few hours. The frequency of the birth of conjoined twins is difficult to estimate, but perhaps 5% of monozygotic (identical twin) twinnings fail to separate completely and are conjoined. "According to Dr. Alan Guttmacher, noted physician and monozygotic twin, conjoined pairs are rarities which occur only once in 50,000 to 80,000 births." There is a great range in the degree of fusion, and depending upon which organs are shared, some twins have undergone separation surgery where both have survived. Points of juncture can be entire torsos, the top or side of the cranium, hips, rear ends, chests.
Sometimes one twin will be more complete an individual fetus and the other will be dependant upon it, or will die during birth and appear as an extra arm or head; a parasite attatched to the autosite. Extreme cases of conjoined twins are those in which one twin is within the other. The cysts generally appear in in body areas where conjoined twins are likely to be joined, namely the liver, brain, and abdominal area. These form tumors in the body cavity and range from complete fetuses to tumors containing teeth and hair and range in size from a tennis ball to considerably larger. One strange case in the early part of the 20th century was that of a boy who was literally "with child." His brother was contained in a sac communicating with the abdomen and connected to the side of the cyst by a short umbelical cord. The fetus made its appearance as a tumor when the boy was nine and subsequently grew too large for the child to contain it, thus killing its host. Another half-developed fetus was removed from the abdomen of a 13-month-old girl in Portland, Oregon in 1940. In 1972 a 6-week old child was found to be carrying its unborn sibling in its abdomen. These cysts can aslo be found growing in adults.