Conjoined twins are twins, almost always identical, who are connected through skin, organs or other parts of the body. Conjoined twins are frequently inaccurately referred to as Siamese twins, probably in part due to Chang and Eng Bunker, twins born in Siam who traveled with P.T. Barnum’s Circus during the 19th century. The term, “Siamese twins,” is inaccurate since conjoining occurs throughout the world at a rate of about 1 in 400,000 live births.

In most cases, on the thirteenth day after a pregnancy begins, twins separate into their own egg sacks, and each twin creates a placenta. When conjoined twins occur, this separation has failed to take place, and researchers are mystified as to the cause. They suggest contributory facts may be environmental or genetic. Classification of conjoined twins rests on the way in which the twins are joined, and this joining exhibits a great deal of variation.

Survival and ability to separate the twins depends upon the type of conjoining. While some conjoined twins can do very well after separation, in other cases, separation may be impossible, or may mean the loss of life of one of the twins. If twins share vital organs, separation is far more difficult, as organs tend not to be split. This is the case with twins joined at the spine as well, as surgery can frequently cause paralysis or death.

In some cases, conjoined twins have one healthy twin, and one very unhealthy, called parasitic. In these cases, separation is necessary to protect the healthy child. It is frequently tragic and difficult for parents to make such decisions, since agreeing to separation surgery is essentially agreeing to allow one child to die. On the other hand, the choice not to separate parasitic twins tends to mean both twins will die.

When conjoined twins cannot be separated they frequently are able to manage through living a very different lifestyle. Difficulties occur when the twins share the same reproductive system, or excretory pathways. Others suffer from postural difficulties if they are joined at the head. Life expectancy for conjoined twins who cannot be separated frequently depends on how many organs are shared. If heart, lungs, liver, or kidneys are shared, providing the work for two bodies may be very difficult. In some cases, the conjoining is almost always fatal during the first few days of life.

Conjoined twins who share a heart have a low rate of survival. Twins who share a heart and brain tend to be considered non-viable. In other cases, called inclusion twinning, one twin absorbs the body of a dying twin while still in utero. Sometimes the surviving twin must undergo surgery to removed skeletal portions of the other twin’s body.

Literature and film have always been fascinated by conjoined twins. Mark Twain wrote a novel and several short stories regarding conjoined twins. Conjoined twins were often exhibited as “freaks” in circus shows. The recent film, Stuck on You starring Matt Damon and Greg Kinnear was a humorous but insightful portrait of conjoined twins.

Separation attempts seem most successful in Saudi Arabia, where 9 separations have been performed in the last 15 years with a 100% survival rate. Usually doctors will not undertake separation without the possibility of at least one of the children surviving the surgery. The courts have sometimes stepped in, as in England, where a judge ordered the separation of two children despite the parents’ strong objection. The two twins were parasitic and the weaker twin did die. It is hoped that with greater understanding, and perhaps with fetal surgery techniques, rate of survival in separation may increase, offering conjoined twins the same chance as other infants to live a healthy and productive life.