Conjoined twins often capture the public's attention when the usually risky separation surgery takes place. Parents and caregivers must consider the benefits and risks to one or both of the babies, and make some very heart-rending, ethically challenging decisions. Is individuality important enough to risk the lives of both children? If one child will be put at more risk, should the surgery be attempted?

What many people don't realize is that most parents of conjoined twins will never have the opportunity to weigh these options, because fifty percent of conjoined twins do not survive birth. Of those who do, another twenty-five percent or more will not survive to adulthood. In the era of ultrasound, many of these parents face another painful decision--whether or not to continue a pregnancy that carries a higher risk to both mother and babies.

Conjoined twins are formed much like other identical (monozygotic) twin pairs. One sperm fertilizes one egg, and for reasons unknown, this egg splits into two distinct individuals. Twins become conjoined when this split happens after the twelfth day of development. Conjoined twins are also predominately female and occur more frequently in Asia than in the West.

Twins are conjoined in many different ways. Here are the most common:

Up to 40% of conjoined twins are Thoracopagus twins,connected in the thoractic region (chest). Twins share a heart and as a result often don't do well, and can never be separated without at least one of the twins dying. Another 35% of conjoined twins are Omphalopagus, connected at a little lower. Each twin has its own heart, but they often share other vital organs, which again makes separation risky or impossible. Another 19% of conjoined twins are Pygopagus, connected back to back, to the buttocks. The possibility of separating these twins depends on the extent to which their reproductive systems, intestinal systems, nervous systems and spinal columns are connected, as separation may cause paralysis or other complications.Other types of conjoined twins exist, but are quite rare.

Modern medicine has made it possible to separate some conjoined twins successfully. However, this area of medicine is still in its infancy, as there are so few conjoined twins who are good candidates for separation, and so few doctors who have had the opportunity to specialize in this area as a result. As time goes on, we will probably see more developments in this area of medicine, but the ethical questions surrounding the separation of conjoined twins will be forever with us.