Firstly, the correct term for twins born in this nature is conjoined twins, not Siamese twins. The term comes from a pair of male conjoined twins named Chang and Eng Bunker born in 1811, who were perhaps the most famous conjoined twins of there time. Being from Siam, now know as Thailand, they were known as the Siamese twins and others having the same condition were referred to in the same manner.
To understand conjoined twin, one must first be familiar with the mechanisms behind identical twins. When a fertilized human egg splits between day 1 and 13 of fertilization, it will develop into identical twins. Generally the earlier the separation the better the twins chance for survival.
Scientist are not completely sure why the egg splits in the first place, but, apparently if the division is for some reason delayed beyond the 13th day the separation will not be complete and conjoined twins will formed.
Experts on conjoined twins have theorize that by 13 days, the natural affinity that cells have for their own kind is developed enough to interfere with successful division. That is to say that, cells of the same type will stick with one another so as to form organized communities of cells of the same function. This could mean that if the force causing the egg to split is not thorough and well carried out; some cells will continue to adhere to one another at the point of division. This is illustrated in the fact that these types of twins are never conjoined at different parts of the body; they always mirror each another. There are no documented cases of twins connected back to chest, head to arm, chest to stomach etc.
Another theory is that when the egg divides, the separation is complete although late, causing the two embryos to be distinctly individual but because of how late the division, the embryos share amniotic sacs and are very close to each other. At about three weeks a problem occurs.
At this stage the mass of cells that each embryo has grown into now has to roll together to form a neural tube that will become the central nervous system. However to do this, the edges must send out signals to attract each other to form the tube. Scientists speculate that when a twin gets in the way these signals may attract the other tube as well. This may be the case of twins who are conjoined at the brain.
The severity of the union is determined by the organs (if any) that are shared and how life altering the point of connection will be. For example, twins connected at the top of the head will be much more challenged than twins such as Chang and Eng Bunker who were connected at the side, sharing the liver; a particularly regenerative organ. The area of the body in which they are connected can pose great obstacles for surgeons who venture to separate them and in some cases such as dicephalus twins who are twins that share one body but with two distinct heads, separation is impossible. However new advances in medical imaging and surgical techniques are improving the prognosis of conjoined twins who are candidates for separation and new equipment is improving the out look on life for those who aren’t.