“Siamese twins” is an older term for conjoined twins, identical twins whose bodies are physically connected at birth. The name comes from Chang and Eng Bunker, conjoined brothers born in Siam (now Thailand) in 1811, who became internationally famous. Today, the medical community uses “conjoined twins” instead, as the older term ties a medical condition to an ethnicity. Conjoined twins are rare, and the specifics of where they’re joined, what organs they share, and whether separation is possible vary enormously from case to case.
How Conjoined Twins Form
Conjoined twins always develop from a single fertilized egg, making them identical twins who share the same DNA. Beyond that basic fact, scientists have debated the exact mechanism for over a century, and two competing theories persist.
The older explanation, called the fission theory, proposes that the embryo begins to split into two identical twins but fails to fully separate. This incomplete split is thought to happen around days 15 to 17 after fertilization, when the embryo has already formed a two-layered disc. Because the split starts late and doesn’t finish, the twins remain partially connected.
The newer and increasingly favored explanation works in the opposite direction. It suggests that two separate embryos form early in the first week after fertilization, but because they develop so close together inside the same sac, they physically grow into each other and fuse. Where on the body they fuse depends on timing: fusion at one end produces twins joined at the head, fusion at the other end produces twins joined at the lower spine, and fusion in the middle produces the more common chest or abdominal connection. Most current research papers support this fusion theory, though textbooks still commonly teach the fission model.
Where Conjoined Twins Are Joined
The connection point defines nearly everything about a conjoined twin pair, from which organs they share to whether surgical separation is even possible. The most common types, accounting for the large majority of cases, involve the front of the body.
- Thoracopagus (chest): The most common type overall, representing about 42% of cases when combined with chest-and-abdomen connections. These twins face each other and almost always share some portion of the heart.
- Omphalopagus (abdomen): Joined at the belly area, often sharing the liver or parts of the digestive tract, but not the heart. This accounts for roughly 5.5% of cases.
- Parapagus (side by side): Joined laterally, often sharing a pelvis. One subtype, where twins share one trunk but have two separate heads, makes up about 11.5% of cases.
- Craniopagus (skull): Joined at the head, sharing the membrane layers around the brain but rarely the brain tissue itself. About 5.5% of cases.
- Pygopagus (lower back): Fused at the sacrum and buttocks, typically sharing a single anus but having two separate rectums.
- Ischiopagus (pelvis): Joined at the lower abdomen with fused pelvic bones.
A small percentage of cases are asymmetric, meaning one twin developed incompletely and is essentially a partial body attached to a fully formed sibling. These are sometimes called parasitic twins.
How They’re Detected Before Birth
Conjoined twins can be suspected on ultrasound as early as 7 weeks of pregnancy, though a reliable diagnosis typically happens around 10 weeks using transvaginal ultrasound. One early clue is seeing two embryos but only one yolk sac, or finding no dividing membrane between twins at any point during the pregnancy. All conjoined twins share a single chorion and a single amniotic sac, so the absence of a separating membrane is a key red flag.
Once conjoined twins are identified, detailed imaging maps out exactly what the twins share. Echocardiography checks whether their hearts are fused, and additional scans examine the digestive, urinary, and skeletal systems. This information shapes every decision that follows, from delivery planning to whether separation surgery could be an option.
Chang and Eng: The Original “Siamese Twins”
Chang and Eng Bunker were born in May 1811 in Meklong, a fishing village in what was then Siam. They were connected by a thick band of tissue at the chest that contained shared blood vessels and an artery, but their internal organs were otherwise separate. They were eventually brought to the United States, where they toured as a public spectacle before settling in North Carolina, marrying two sisters, and fathering a combined 21 children. Their fame was so widespread that “Siamese twins” became the default phrase for the condition for well over a century. The term stuck in everyday language even as medicine moved on.
Separation Surgery
Whether conjoined twins can be surgically separated depends almost entirely on what they share. The single biggest factor is the heart. When twins share a fused heart, separation is generally not survivable because the heart cannot be divided between two people. If prenatal imaging reveals cardiac fusion early in pregnancy, some families and medical teams consider ending the pregnancy rather than continuing to a birth with very limited options.
When the heart is not shared, the picture improves considerably, though separation still involves major challenges with shared livers, intestines, or blood vessels. Timing matters too. One study found a 50% survival rate when separation was attempted during the newborn period, but that rate climbed to 90% when surgery was delayed until after 4 months of age, giving the twins time to grow stronger and better tolerate the procedure.
Separation often requires planning for organ reconstruction or even transplantation. In cases where the liver cannot be divided, for example, the mother may be evaluated as a living donor for a partial liver transplant. These surgeries involve large teams of specialists and can last anywhere from several hours to more than a day.
The Ethics of Separation
The hardest cases arise when one twin cannot survive without the other. If both twins depend on a single shared heart, separating them means one twin will die so the other can live. Medical ethics guidelines generally hold that intentionally causing the death of one person to save another is not permissible, even when doing nothing means both will eventually die.
There’s another dimension that makes these decisions unusual. When adult conjoined twins have been asked whether they would want separation if it meant their twin might die, they have overwhelmingly said no. They preferred to remain conjoined. This is striking because decisions about infant conjoined twins are made by parents and doctors, not the twins themselves. Some ethicists argue that the expressed preferences of older conjoined twins should carry real weight in these cases, since no one else can truly understand what it means to live in a conjoined body.
The result is that separation surgery is not automatically the goal. For some conjoined twins, the safest and most ethical path is to remain connected and receive medical support for the complications that arise from their shared anatomy.
Living as Conjoined Twins
Conjoined twins who are not separated, whether by choice or because separation is too risky, can and do live full lives. Chang and Eng Bunker lived to age 62. Modern conjoined twins have attended school, held jobs, and navigated daily life with remarkable coordination. Each pair develops their own system for movement, sleeping, and decision-making, adapting to a physical reality that is genuinely unique to them. The degree of independence each twin has depends on the connection point: twins joined at the head face different challenges than twins joined at the abdomen, and pairs who share legs must coordinate every step they take.