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Conjoined Twins Causes, Surgery Separation, and Survival Rate

Reviewed on 2/21/2020

What Are Conjoined Twins (Picture)?

An X-ray of conjoined twins.
One of the causes of conjoined twins is when twin embryos in the egg do not separate completely in the womb.

Conjoined twins (formerly known as Siamese twins) are two babies (twins) born physically connected to each other. Conjoined twins may be joined at one or more sites in the body. They are usually classified according to the body areas that are joined. They may share certain critical internal organs like a brain or heart. In other cases, each twin may possess all necessary organs and the area of joining may be small. 

Conjoined twins may be joined at the head, chest, spine, abdomen, trunk, or a combination of these.

What Causes Conjoined Twins to Form Together?

  • Conjoined twins are created early after fertilization of an egg (conception).
  • Sometimes a fertilized egg divides to create identical twins.
  • One theory is that if this division process occurs late, conjoined twins may be formed.
  • Another theory is that a fertilized egg splits in two but the two embryos later fuse together.
  • Some conjoined twins can marry, however; it is difficult because it depends on which organs they share, and those they do not, for example, Abby and Brittany Henson.

Are Conjoined Twins Always the Same Gender? How Common Are They?

  • Conjoined twins represent about 1% of all cases of identical twins.
  • In the United States, conjoined twins occur in 1 per 33,000-165,000 births and 1 per 200,000 live births.
  • About 40% to 60% of conjoined twins are stillborn.
  • Living conjoined twins are more likely to be female, with a female-to-male ratio of 3:1 or greater among conjoined twins.

What Are the Types of Conjoined Twins?

Each set of conjoined twins is unique. Doctors have developed ways to describe of classify conjoined twins based upon their anatomy. There are two main categories of conjoined twins. Symmetrical or equal conjoined twins have two well-developed bodies. Asymmetrical or unequal conjoined twins are cases in which a small part of the body is duplicated, or an incomplete twin body is attached to a fully developed twin body. 

Another way to classify the type of conjoined twins is based upon the anatomic location of the fused body parts:

  1. Thoracoomphalopagus: joined at the chest, abdomen, or both. This group is the most common group of conjoined twins and represents about 74% of cases.
  2. Thoracopagus or xiphopagus: joined at joined at the chest
  3. Omphalopagus: joined at the abdomen.  
  4. Pygopagus: joined at the buttocks
  5. Ischiopagus: joined at the ischium, one of the hip bones
  6. Craniopagus: joined at the head 

During Surgery, Are Conjoined Twins Separated Together or Separately?

  • Surgery can be performed to separate some types of conjoined twins.
  • The prognosis for conjoined twins who undergo surgical separation is highly individualized and is dependent upon the degree and severity of the condition such as the presence of shared organs. 
  • The type of surgery depends upon the areas of the body that are joined, and multiple specialists are typically needed to plan and execute the surgery.

Can One Conjoined Twin Survive the Other? What Is the Survival Rate?

The prognosis and life-expectancy for conjoined twins is more guarded for conjoined twins who share critical organs. In severe cases, stillbirth often occurs, or surgical separation may not be possible (for example, twins that share a heart or brain).

Although many  conjoined twins die before or shortly after birth, other conjoined twins can survive with or without surgical separation. Some twins can survive while their twin may not; each conjoined set of twins is unique.

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Reviewed on 2/21/2020
References
Kamal, K, MD, et al. Conjoined Twins. Medscape. Updated: May 08, 2018.
<https://emedicine.medscape.com/article/934680-overview>

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