Siamese or Conjoined Twins

Luckily, most twins are born healthy. But in some situations twin foetuses develop differently. A familiar example of differently developing twins is so called Conjoined twins (Siamese twins). Conjoined twins are twins with connected bodies, varying from a small piece of connective tissue to one or more shared body parts. Some conjoined twins can be separated from each other through surgery. Whether or not this is possible will depend on the degree of connective tissue, and in particular the extent to which the internal organs are being shared with one another. Sometimes even parts of the brains are fused. Because of the high risks, surgery often leads to medical and ethical problems.

Chang and Eng: ‘Siamese twins’

The name Siamese Twins is derived from conjoined twins Chang and Eng Bunker (1811-1874) from Siam, nowadays called Thailand. Chang and Eng were born with attached body parts: the sides of their abdomens were attached to each other by a tube with a length of 22 centimeters and a diameter of 4 centimeters. With current surgical techniques, they could easily have been separated, but at that time this was impossible . Because of their striking appearance they received worldwide fame as circus performers. They got married to the sisters Sallie and Adelaide Yat and together they had 21 children. One morning after waking up, Chang discovered that Eng had died beside him in bed, and after several hours he was also deceased.

The occurrence of conjoined twins

Siamese twins are genetically identical and thus always of the same sex. They develop from the same fertilized egg and have a shared amniotic sac and placenta. Siamese twins are created when the fertilized egg splits too late. Normally, a fertilized egg splits into two separate embryos within 12 days after conception. If the split occurs within 3 to 4 days, the embryos have separate placentas. In most cases the egg splits in between day 4 and day 8, leading to a shared placenta. If the separation occurs later than 8 days, various complications can occur, like shared body parts.

There are different types of Siamese Twins:

  • Thoraco-omphalopagus (28%): the bodies are fused at the height of the chest. These twins usually have a shared heart. Sometimes they also have a common liver or a piece of the digestive tract shared;
  • Thoracopagus (18.5%): upper bodies are fused to the abdomen. The heart is always involved in these cases;
  • Omphalopagus (10%): upper bodies are fused together below the chest. In this case, the two bodies have separate hearts. However, there are shared organs, such as the liver, the diaphragm or a part of the digestive tract;
  • Parasitic twins (10%): Siamese twins in which the bodies are not equally developed. One of the bodies is less developed, and therefore depending on the other body in order to survive;
  • Craniopagus (6%): the bodies are separated, but the skulls are connected. The skulls can be fused in several ways: at the back of the head, the front of the head, or the side of the head, but not on the face or the base of the skull.

Other, rare, forms include:

  • Cephalopagus: Siamese twins with two faces, one on either side of a joint head. The top of the body is shared, while the bottom of the body is separated. These twins usually do not survive because of the fusion of the brains;
  • Synecephalus: twins with a single head and face with four ears and two bodies;
  • Cephalothoracopagus: these twins have a shared head and torso. There are two faces on different sides of the head, or sometimes a single face with an enlarged head;
  • Xiphopagus: upper bodies are connected between the navel and the breastbone. In most cases, there are no shared organs except sometimes the liver (like Chang and Eng);
  • Ischiopagus: the lower part of the bodies is shared, with the ends of the backbone of the two bodies are connected in and angle of 180 ˚. These twins have four arms, three or four legs and a shared anus and genitals;
  • Omphalo-Ischiopagus: these are connected like the Ischiopagus twins, but with the face towards each other and a shared (lower) stomach;
  • Papapagus: twins side-by-side fused with a shared (lower) stomach. Dithoracic parapagus twins have a shared pelvis and a shared (lower) stomach, but a separate upper body. Diprosopic parapagus twins have a shared torso and one head with two faces. Dicephalic parapagus twins have one torso, two heads and one, two or three arms;
  • Craniopagus papsiticus: As craniopagus, but with a second head at the other head is grown together;
  • Pygopagus (Iliopagus): two bodies back-to-back are grown near the buttocks.

How common is it?

The birth of conjoined twins is rare: about one in 50,000 to 100,000 births. Yet we even fewer of them, because most (40-60%) conjoined twins are stillborn or die within a day (35%). The survival rate is somewhere between 5 and 25%, though even then many conjoined twins do not make the first year.

Most surviving conjoined twins are female. According to 600 publications from the last 50 years about 70% of all conjoined twins were female and about 20% were male. Of the remaining 10%, the sex was unknown. The reason why there are more female Siamese twins is not entirely clear. Scientist think that it has to do with a greater vulnerability during pregnancy for the male sex. Also there might be an increase of the Turner Syndrome in which the Y chromosome disappears and a boy as a girl is born (X0 instead of XY). Another possible explanation is that for girls it might be more common that the egg splits too late.

Quality of life

After birth, parents and physicians have to deal with the complex question whether and when it is possible to separate the children. These choices are not only of medical relevance, but also an ethical problem. Each separation has inherent risks. The risk of complications is usually great and there is a low likelihood both children will survive. In some cases this means that the weakest child’s life must be sacrificed if the conjoined state of living together is unsustainable. The ethical questions obviously have to deal with the the quality of life of the children. Parents and doctors can have different views on that.

The separation of Siamese twins

Because the surgical separation of Siamese twins is often a very risky procedure, a careful balance must be made regarding the health risks of the twins. Doctors will have to try and estimate what will be the best moment for the children to be separated. Survival rates increase as the children grow older, and usually we wait until after the first six months to do the procedure. Sometimes it is not possible to wait this long, for example if one of the children dies, or when the health of one or both children is endangered.

Modern techniques, such as ultrasound, have increased the likelihood of success of separation surgery. It is of vital importance to know exactly which organs are shared. The liver is the only internal organ that can be split between the two children. This is not possible for any of the other organs. The survival rate after surgery depends on the type of Siamese twins. For example, for twins with connected backbone there is a 68% chance of successful separation, while there are no known cases in which twins with a shared ventricle are successfully being separated.

After separation, most twins need intensive rehabilitation. Often their spines are misaligned and many twins have a hard time bending over or sitting upright. They literally need to learn to stand on their own feet.


Segal, viz (1999). Entwined Lives. Twins and What They tell us about human behavior. Plume, New York.

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