Conjoined Twins

rate of conjoined twins

Conjoined twins are identical twins who are joined together somewhere in the body, most often at the chest, abdomen or pelvis. They are monozygotic twins, in the sense that they share the same zygote.

The formation of this twin type is believed to be the result of late twinning. When the twinning occurs more than 12-13 days after fertilization of the egg, it may lead to the formation of conjoined twins because the embryo may not split completely.

Conjoined twin births are very rare, amounting to around one birth in every 100,000 births. The ratio of female and male sets was found to be three to one.  There is a slightly higher chance of conjoined twins from Africa and Southwest Asia.

Depending on where they are joined, some may share some vital internal organs.

Classification of conjoined twins

There are several ways to classify these unique twins.

  • 75 percent are joined at the chest wall or upper abdomen (thoracopagus and omphalopagus)
  • 23 percent are joined at the hips, legs or genitalia (pygopagus and ischiopagus)
  • 2 percent are joined at the head (craniopagus)

While conjoined twins are typically classified by the point of fusion they may also be divided into two broad categories: homogenitally conjoined twins, who shared a single set of genitalia, and heterogenitally conjoined twins, with two distinct sets of genitalia.

Conjoined twins, sometimes referred to as Siamese twins, named after the famous conjoined twins, Eng and Chang Bunker from Siam.  Chang and Eng were joined at the torso by a band of flesh, cartilage, and their fused livers. In modern times, they could have been separated.

Survival rates

Conjoined twins rarely survive because of the complexity of the bodies. 40% are stillborn or die within twenty-four hours after birth (35%).

Conjoined twins can only be separated surgically if none of the vital organs are involved.  The success of the separation surgery depends on many factors, mainly where the twins are connected and which structures they share.  Only 60% of the surgically separated cases survive.

Some people consider the separation as unethical if it involves death or disability of one of the twins. Those that extensively share vital organs can generally not be separated as separation might lead to the death of one or both twins. This presents some ethical dilemmas in the surgical management of conjoined twins, but the overall decision still lies with the parents.

Conjoined twins, if they survive, can lead healthy lives. There are instances where some have even married and become parents. The famous Siamese twins, Chang and Eng Bunker, fathered twenty-one children in thirty-one years.

Other kinds of conjoined twins

There are also some rare kinds of conjoined twins, such as parasitic twins (where one twin is not completely formed and depends on the other twin to sustain life), and fetus in fetu (where one twin’s fetus is present inside the body of the other twin).


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