Conjoined Twins

Conjoined twins are identical twins whose bodies are physically joined together while still inside their mother’s uterus. There are many different ways in which twins can be joined; some are joined at the head, others at the side or torso, and still others along the spine or pelvis. Conjoined twins have been reported throughout history, but they are very uncommon, with statistics suggesting that only one in 50,000 to one in 100,000 pregnancies will produce conjoined twins, with some medical sources reporting about one in 50,000 to one in 200,000 births. In the nineteenth and twentieth centuries, conjoined twins were also known popularly as Siamese twins. This is because a pair of conjoined twins from Siam (now known as Thailand), Chang and Eng Bunker, became famous through public exhibitions in the nineteenth century. The world-famous circus owned by P. T. Barnum billed them as the renowned Siamese Twins (a term that was already in use to describe the conjoined twins), thus establishing the term to describe any set of conjoined twins.

Brief History

There are two different theories that try to explain how conjoined twins are produced. One of these is based on stem cells and the way they behave during the very earliest stages of fetal development. Stem cells are produced as a fetus begins to develop, and they possess several unusual properties. One of these properties is that stem cells are able to develop into other types of cells; they are akin to proto-cells that eventually form into different subgroups and acquire specialized functions. One subgroup of stem cells might develop into kidney tissue, while another might develop into brain tissue. Another interesting property stem cells have is that they seek out cells similar to themselves—obviously, this helps them form groups of cells that are similar enough that they can all adapt to whatever function is needed. The stem cell theory of conjoined twins proposes that shortly after a woman’s egg is fertilized by a man’s sperm, the fertilized egg begins to form identical twins, but does not separate completely. Ordinarily, this would result in a traditional set of twins, with identical characteristics but no physical connection. Another theory proposes that, in some cases, when two fertilized eggs are formed, the stem cells in each fertilized egg might attract the stem cells in the other fertilized egg, bringing the two eggs together and causing them to partially fuse, although the exact mechanism remains uncertain. This is sometimes called the fusion theory of conjoined twin development.

The competing theory of conjoined twin development is known as the fission theory. Under the fission theory, an egg is fertilized as described in the fusion theory, and as in that theory, the fertilized egg splits. However, the split is not complete as it would be in the case of a conventional set of twins. Instead, the fertilized egg splits only partially, and the remaining connection is responsible for the physical connection present when the conjoined twins are born. It is estimated that approximately half of all conjoined twins are stillborn, meaning that they do not survive childbirth. Of those conjoined twins who do survive childbirth, some are not able to survive outside of their mother’s womb because of the nature of their physical connection to their twin. For example, some conjoined twins have lungs that are unusually small or partially fused; in the womb, this is not an issue because blood and oxygen are supplied by the mother, but once they are born, their lungs cannot sustain them. A 2023 multiomics study stated that although the exact cause of conjoined twinning remains uncertain, new research has started to examine molecular differences in conjoined twin fetuses to better understand how the condition develops.

Overview

Some conjoined twins can easily be separated through surgery, allowing both twins to live separate lives. For other twins, this may not be an option, or it may be one fraught with great risk, as in cases where conjoined twins are connected by vital organs such as the liver, heart, or brain or when they share major blood vessels. These types of conjoined twins must remain connected if they are to survive, and even so, their lifespan may be shorter than typical. In very unfortunate cases, surgery to separate conjoined twins may be necessary in order to save their lives, but it may also require the sacrifice of one twin. This can happen when twins share a vital organ that will not be able to sustain them both over the long term, but which also cannot be divided between the twins in a way that will allow both to survive. This can create complicated ethical and legal dilemmas for the twins, their family, and the medical personnel charged with administering to their needs.

Apart from the many different ways in which conjoined twins may be connected, there are two broad categories physicians use to describe different types of conjoined twins. The first type is symmetrical, which means that two fully developed babies are born with a physical connection between them. The second type of conjoined twin is asymmetrical. This means that one of the conjoined twins is fully developed, while the other is smaller or incomplete. Asymmetrical conjoined twins have a much lower chance of mutual, independent viability, meaning that the most likely outcome is that they will have to remain conjoined throughout their lives, or that if they are separated (through agreement or necessity), only the more fully developed twin will survive. Cleveland Clinic’s 2025 review states that about 45 percent of conjoined twins are live births but that only about 8 percent survive.

Because conjoined twins develop from the same egg and sperm, they have identical DNA and therefore are always the same sex. It is not fully understood why, but female conjoined twins appear to have a much greater likelihood of viability than males, although all fetuses begin as neutral or bipotential embryonic structures and the sex of chromosomes is established at fertilization. A 2025 National Institutes of Health (NIH) review reports that conjoined twins have a higher prevalence of females, with a female-to-male ratio of 3:1. It is worth noting that the phenomenon of conjoined twins is not confined to human beings, but also occurs in other species, including mammals, reptiles, amphibians, and fish. Across species, the most common type of connection for conjoined twins to share is a fusion at the thorax, which may entail a shared heart, liver, or digestive system.

Bibliography

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