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Mount Airy, North Carolina
JUNE 2006
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In Learn More the National Geographic magazine team shares some of its best sources and other information to expand your knowledge of our featured subjects. Special thanks to the Research Division.

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Conjoined twins occur when the zygote of identical twins fails to completely divide. The incidence of conjoined twins in Western countries is one out of every 50,000 to 200,000 births. But in Southeast Asia and Africa, the odds increase to one out of every 14,000 to 25,000 births. So far, the reason for this difference is unknown, although some scientists think the increased incidence of conjoined twins in these regions may be due to genetic causes. Sixty percent of conjoined twins are stillborn and 40 percent of the live births die within a few days. Seventy percent of all conjoined twins are female.

The earliest known case of conjoined twins dates back to 945 when brothers from Armenia were taken to Constantinople for medical evaluation. Mary and Eliza Chulkhurst who were born in Biddenden, England, in 1100 and lived to the age of 34, were the first conjoined twins to be well-known publicly. In their hometown, little cakes imprinted with their image are distributed at Easter time to honor their gift of land to the local church. The first attempt at separating twins was in Germany in 1495, but the surgery ended in the death of both twins. In 1689 in Basel, Switzerland, twin girls joined by a ligament at the sternum were successfully separated by using a constricting band.

Today, one of the most successful places for surgery to separate twins is Saudi Arabia. Since 1990, the medical teams at King Abdulaziz Medical City in Riyadh have successfully separated 12 sets of twins—the most recent being sisters from Morocco on March 7, 2006.

—Marisa Larson

Related Links

Conjoined Twins
The different types of conjoined twins, how they are formed, and the ethics of separation are discussed on this informative site.

Conjoined Twins International
CTI provides support for families of conjoined twins, education for the general public, and aids ongoing medical research in the field of conjoining.


Leroi, Armand Marie. Mutants: On Genetic Variety and the Human Body. Viking Books, 2003.

Quigley, Christine. Conjoined Twins: An Historical, Biological and Ethical Issues Encyclopedia. McFarland and Company, Inc., 2003.

Spencer, Rowena. Conjoined Twins: Developmental Malformations and Clinical Implications. Johns Hopkins University Press, 2002.

Wallace, Irving, and Amy Wallace. The Two: The Story of the Original Siamese Twins. Simon and Schuster, 1978.

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