What Do We Know About Conjoined Twins?

“Jodie” and “Mary,” conjoined twins–“Siamese twins” is no longer a socially acceptable term–were born in Manchester, England, on Aug. 8, attached at the abdomen. Their parents, from Malta, came to England to get more sophisticated medical care for their daughters. Now the parents are deciding whether to appeal the British court ruling that the twins must be separated, causing Mary to die so that Jodie might live. According to physicians, Jodie has a normal brain, heart, and lungs; Mary has none of these and survives only because of her stronger sister. The court concluded this “parasitic” relationship would eventually cost Jodie her life, so she and her sister should be separated. How unusual are conjoined twins, and what do we know about their biology?

Jodie and Mary, like all conjoined twins, are identical. Nobody knows what causes twins to be conjoined, but how it happens is a matter of bad timing. If a fertilized egg is going to split and form a set of identical twins, it must do so by the 13th day of its existence. If the cleavage occurs even one day later, the separation will be incomplete, and the twins will be connected. While the majority of identical twins are male, about 70 percent of conjoined twins, like Jodie and Mary, are female. Again, no one knows why.

The condition is rare and usually fatal. Estimates of incidence vary from one in 50,000 to one in 100,000 births–about one in every 200 sets of identical twins. Approximately 75 percent of conjoined twins die within 24 hours of birth–40 percent of them are stillborn.

Doctors have created numerous systems to classify conjoined twins. These broadly come down to whether the twins are equally developed–the Maltese twins are unequal–and the place at which they are joined. The medical term to describe them ends with the suffix “pagus” from the Greek for “something fixed.” For example, thoracopagus twins, who share a chest wall, are the most common type, about 35 percent of cases. The Maltese twins, fused at the pelvis, are ischiopagus, which occurs in about 6 percent of cases. The rarest type, only about 2 percent, are craniopagus, or joined at the skull. Lori and Dori (she prefers to be called Reba) Schappell, born in 1961, are the only known adult twins joined at the head in the U.S. Dori is a country singer, and Lori has worked in hospitals.

One of the first recorded cases of conjoined twins is also the first recorded case of a failed surgical separation. Male twins born in Constantinople in the 10th century were joined across the trunk. After one twin died, a separation was performed, but the other died three days later. In 1689 the first successful separation was performed in Germany on a pair of twins connected at the waist. But it wasn’t for another 300 years that surgical separation became common. Children’s Hospital of Philadelphia pioneered this work, performing its first separation in 1957. Dr. C. Everett Koop, former surgeon general of the United States, gained international fame for his successful separation of Dominican twins there in 1974. Almost 200 separation surgeries have been performed since the 1950s. One study shows that one or both twins survived the surgery in 150 cases. In the case of Jodie and Mary, one twin will not live, which echoes the famous 1993 case of Indiana twins Angela and Amy Lakeberg, who shared a single heart. Amy was selected to die in the surgery, but Angela lived less than a year.

The now-discredited term “Siamese twins” was coined for Chang and Eng Bunker, who were born in Siam (now Thailand) in 1811 and came to America as young men to be exhibited. They died in North Carolina in 1874. The brothers married sisters and fathered 21 children between them. They were so famous that both sides in the Civil War referred to them as symbols for their cause. The Union argument was that such a pair should never be divided, and the Confederacy said they were unnatural. A thick band of flesh joined the Bunkers across their chests. If they had been born today, the surgery to separate them would be unremarkable.

Explainer thanks Dr. Nancy L. Segal of California State University, Fullerton and author of Entwined Lives. More information on conjoined twins can be found at this Swarthmore College site.

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