What Are the Different Types of Conjoined Twins?

Patient Presentation
While volunteer teaching in the human body curriculum of a local elementary school, the students asked a pediatrician several questions about twins. They were especially interested in conjoined twins since there had been some recent media stories about a set of conjoined twins. He explained that people weren’t sure exactly how conjoined twins happened, but the usual thought was that it was due to cells not splitting apart completely. The students also wanted to know why conjoined twins were sometimes called Siamese twins. He said he didn’t know this answer, but later looked up the answer using the Internet and sent the answer to the children.

Discussion
Conjoined twins (CT) have sparked people’s interest for thousands of years. The Roman god Janus with two heads and faces is one example. The Greek mythological creature the centaur is another example being part human and part horse. The Babylonian god Marduk was a “double-god” with 4 ears and 4 eyes to be able to see and hear everything.

In Kent, England in ~1100 A.D., the Biddenden maids, Mary and Eliza Chulkhurst, supposedly were CT joined at the hip and shoulder and lived for 34 years. After their death they gave land to the local church and biscuits/cakes with their likeness has been given to the poor at Easter in their honor since. One of the most famous sets of conjoined twins were Chang and Eng Bunker. Born in Siam (modern day Thailand) they were thought to give rise to the common term “Siamese twins” for CT. They were joined at the lower chest and livers, lived for 64 years, married sisters and had 21 children between them. They traveled throughout the world as entertainers including working with the famous P.T. Barnum.

The developmental cause of the malformation CT is uncertain with different data supporting a problem with fission (or incomplete clevage along the plane in a single embryo) or fusion (of two separate embryos). The prevalence is estimated to be 1;50,000 pregnancies but 1;200,000 live births. There is great variation, but it appears that CT are more common in females and in some places in the world including South America. Survival is low and many die in the early natal period or as part of surgical separation. As with all people, each set of CTs and each person within the set is unique. The exact location, organs involved, circulation status to the organs and many other factors help to determine survivability as well as the possibility of attempting surgical separation. A multidisciplinary team approach to surgical separation is necessary including extensive pre-surgical radiologic imaging and planning as well as ethical considerations.

Learning Point
The types of conjoined twinning are usually noted by the union site with the suffix “pagus” attached. Pagus means fixed or solid.
The table below lists the CT types, union site, primary shared structures and the incidence from the International Clearinghouse for Birth Defect Surveillance and Search in 2011.
See also line drawings of different types of CT.