The first known case of Siamese goes back to 1811. Chang and Eng Bunker, born in Thailand (then named Siam, that explains the name of malformation), were connected by the thorax. Extremely lucky, they lived 63 years. Today, the causes of malformation are not completely known, and few operations succeed.
In medicine, the Siamese ones are considered before just like twins whose gestation turned badly. The rate of births of twins in France since the beginning of the century borders each year 1%, including 0,04 % of identical twin. The proportion the Siamese one accounts for approximately 1 birth out of 75 000, that is to say 0,03 % of identical twin. In 70 % of the cases, the Siamese ones are linked by the thorax, in 18% by the sacrum (bone of the spinal column), 6% by the basin and 2% by the head.
The birth of identical twin is due to a cleavage of the agg-cellule after fecundation. Normally, this cleavage takes place one or two days after fecundation, and the two embryos have each one an amnion* and a chorion** But if it occurs between the third and the seventh days (approximately 70 % of the twins), the two embryos have same the chorion, but have each one their own amnion, which does not involve harmful consequences. If cleavage takes place as from the eighth day, then the two embryos divide same the chorion as well as the same amnion, and they are thus separated by no membrane. The problems occur when cleavage takes place even longer afterwards : the two embryos are not separate any more and there are the Siamese ones.
The future of Siamese ones is very compromised. The majority, operated or not, don't survive childhood. Moreover, the operations are difficult. Because very often, the Siamese ones are connected only by one bone or of the skin, but also by nervous fabrics and blood vessels, which makes their separation extremely complicated since it is necessary to ensure each one a correct operation of the respiratory and nervous systems. Moreover, certain Siamese have only a heart or a brain for two, which makes their separation strictly impossible. In much of country, the IVG is authorized unbounded time in the case of the Siamese one.
Today, the operations are often turned in a media event. We had the example of Maria Teresa and Maria Jesus Quiej Alvarez, who were born on July 25th, 2001 in Guatemala. They was linked by the cranium (but fortunately not by the brain) and were successfully operated in Los angeles in July 2002. More recently, there was the case of Ahmes and Mohamed Ibrahim, egyptian, who were linked by the cranium too and were operated on Octobre 12th, 2003 in Dallas.
In the beginning of November 2007 in Bangalore (India), 30 doctors operated during 27 hours a two-year-old girl. The development of her siamese sister stopped in the uterus. It caused the birth of a baby with four arms, four legs, four kidneys, two chests, two stomacs and two linked vertebral columns. An operation with a very high risk but without it, the child wouldn't probably had survived more then a few years...

* amnion : pocket surrounding the embryo and containing the amniotic liquid, a liquid allowing the embryo to develop in aquatic environment (because the mammals result from the evolution of certain Amphibians). It is the puncture of the amnion which causes the « loss of water ».
** chorion : membrane external of the embryo, which ensures the contact with the uterus and makes it possible to nourish the embryo.