Siamese Twins Facts

Many people are under the misconception that Siamese twins, better known as conjoined twins, are a rare occurrence. Records of conjoined twins have been around since the 12th century, though the term Siamese twins did not come around until the 19th century. Today, a lot more is known about conjoined twins.

Frequency

• Contrary to popular belief, Siamese twins are not rare. The birth rate for conjoined twins is 1 out of every 200,000 live births, making them significantly more common than previously believed. It is more common for male twins to conjoin in the womb than females. For unknown reasons, female twins have a higher chance of survival, according to the University of Maryland Medical Center.

Survival Rates

• It is estimated that up to 60 percent of conjoined twins do not survive and are stillborn. Up to 35 percent survive birth, but do not live longer than 24 hours. The survival rate for conjoined twins is relatively low, estimated to be between 5 and 25 percent, according to the University of Maryland Medical Center.

How It Works

• Since they develop from a single fertilized egg, Siamese twins are genetically identical. There are no male and a female conjoined twins. With Siamese twins, the egg begins to separate, as it does for identical twins. At some point during the separation, however, the egg stops separating but continues to develop. This lack of completion in separation results in conjoined twins.

Origin of Siamese Twins

• The origin of the term Siamese twins stems from a pair of conjoined twins named Chang and Eng. The men were born in the area of Siam, which is now Thailand. They were born to a half Chinese mother and a full Chinese father in 1811. During their lifetime, they were referred to as the Chinese twins. Today, Chang and Eng are among the most well-recognized set of conjoined twins.

Conjoining Locations

• Siamese twins are joined in various locations. The rarest location is the head, which is known as craniopagus twins, and accounts for only 2 percent of all conjoined twins. Conjoined twins are also thoracopagus, meaning they share a heart. Thoracopagus twins account for up to 40 percent of all conjoined twins. Other locations of conjoining include hips (ischiopagus or parapagus) and abdomen (omphalopagus). Twins conjoined by a vital organ, such as the heart, generally are not separated because of the extreme risk of death. If no vital organs are shared, however, separation is attempted and is sometimes successful.

World Distribution

• Though conjoined twins occur anywhere in the world, they are more likely to occur in India or Africa than in China or the United States. Fewer than a dozen conjoined twins live in the United States, according to Twins World, a website dedicated to twins of all kinds.

Earliest Recorded Siamese Twins

• One of the earliest known recorded cases of conjoined twins dates back to 1100. Mary and Eliza Chalkhurst were born in Biddenden, England, in the area of Kent. In 1134, the twins passed away within hours of each other. The twins were noted as having left behind 20 acres of land to church wardens.