Conjoined twins Tinashe, left, and Tinotenda underwent separation surgery at Toronto's Hospital for Sick Children in 2005. (Hospital for Sick Children)

Conjoined twins are rare, but they frequently enter the media spotlight and spark debate. Questions arise as to whether the twins should be separated from the ethical point of view, and whether the procedure is medically safe.

Twins form in the womb when an egg splits into two. But if the split happens beyond a certain period — for example, in the 12th day — then the cells do not fully divide. Most conjoined twins are female.

Conjoined twin births happen about once every 200,000 live deliveries. Delivering the babies can be a difficult and delicate procedure. About 40 per cent of them are stillborn and of those that do survive, less than half live long enough to be separated.

Certain types of conjoined twins are easier to separate, depending on how many vital organs they share. The first such successful operation happened in the 17th century. Currently many hospitals, including Toronto's Hospital for Sick Children, have conducted numerous surgeries, successful and unsuccessful.

Dr. John Templeton, an expert on conjoined twins in Pennsylvania, told the CBC News show The Current that the separation is followed by a bigger challenge, which is reconstruction. "It may take two to three hours for separation, but about six to eight hours for reconstruction."

Alice Dreger, the author of One of Us, a book on conjoined twins, told The Current that if surgery is to take place, it should happen when the patients are very young, when they heal faster and before their psychological identity is established.

Risks

There is, of course, a risk that one or both twins could die as a result of the operation. In recent times, a set of twins from Iran, Ladan and Laleh Bijani, died in 2003 after the adult women underwent surgery to separate them at the head.

In another highly publicized case, a British court ordered in 2001 the separation of twins so that one, Gracie Attard, could survive. The other, Rosie, died.

But before surgery comes the debate on whether it is necessary, even with the medical capability of success. Dreger says she has looked at conjoined twins and found that most don't have a problem being joined with their twin.

"What I found over and over is that they described their lives being at high quality; in fact, most of them found benefits," she said. "They couldn't imagine themselves singletons."

It's believed that more than 450 sets of conjoined twins have survived, most of them born in India or Africa. It's also believed that the birth of conjoined twins happens in one out of every 200 births of identical twins.

Timeline of conjoined twins

'Scottish brothers'

In the 15th century, these twins were said to be performers in the court of King James III. The twins were fused in one body and lived to about the age of 28.

Helen and Judith

These two girls were born in Szony, Hungary, in 1701. According to literature documenting the two, one of the twins, Helen, was larger than her sister Judith. They lived until they were 21, dying within minutes of each other.

Chang and Eng Bunker

These twins are perhaps the best known. Chang and Eng were born in 1811 in Siam (hence the term Siamese twins). They were joined by cartilage at their sternum and each had two arms and legs. They were brought to the United States and toured with P.T. Barnum. Eventually they settled in the United States, married, became farmers and lived into their 60s.

Modern-day timeline

Daisy and Violet Hilton

The girls were born in Brighton, England, in 1908 and were joined at their hips and buttocks. They toured in Europe and the United States. They later sued their managers and left sideshow amusement acts to pursue vaudeville entertaining. They lived into their 60s.

Lori and Reba Schappell

These twins, born in 1961 in Pennsylvania, are joined at the head. Reba is a country singer while Lori works in a laundry. The women have appeared on television and say that they can live their lives just fine together.

Tinashe and Tinotenda Mufuka

In 2005, Toronto's Hospital for Sick Children separated the twins from Zimbabwe while they were less than a year old. It was the 10th operation to separate conjoined twins by the hospital.

Krista and Tatiana Hogan

In October 2006, Krista and Tatiana Hogan were born in a Vancouver hospital with a combined weight of 12.69 pounds. The tops of the girls' skulls are joined, making them the country's only craniopagus twins. The twins were released from hospital in December 2006. Doctors are expected to decide in August if the girls should be separated.

Some types of conjoined twins

• Thoracopagus: About 35 per cent of conjoined twins are joined in the upper half of the trunk.
• Omphalopagus: Another common form (30 per cent) share an anterior connection of the trunk, or lower chest. These twins usually have four arms, four legs and two pelvises and may share some functions such as the liver.
• Pygopagus: Twins who are joined by the buttocks represent about 19 per cent.
• Craniopagus: Twins who are joined at the head are very rare, with only about 2 per cent of conjoined twins with this type. The Canadian twins born in October 2006 are of the craniopagus type