Children's Hospital Colorado
Colorado Fetal Care Center

Conjoined Twins

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What are conjoined twins?

In rare cases, twins develop so closely that their body parts are joined together. Conjoined twins can be connected through a variety of body parts, including the head, chest, abdomen, pelvis and buttocks. Twins joined at the chest or abdomen are the most common type of conjoined twins, comprising 75% of all cases.

Common types of conjoined twins

The most common sites for twins to be joined are:

  • Thoracopagus - Joined at the chest and facing each other
  • Omphalopagus - Joined at the abdomen and facing each other
  • Pygopagus - Joined at the buttocks and perineum, and facing away from each other
  • Ischiopagus - Joined with a single bony pelvis and four normal lower extremities
  • Craniopagus - Joined at the skull with or without brain connection

What causes conjoined twins?

Identical twins occur when a single fertilized egg (embryo) splits and develops into two individuals. The dominant theory on the origin of conjoined twins suggests that when the single embryo splits later, separation stops before the process is complete, leaving the babies joined. Alternatively, another theory holds that two separate embryos fuse in early development. What would cause either scenario remains unknown.

The exact rate of conjoined twins is not known, but estimates have varied from 1 in 25,000 to1 in 80,000 births. Maternal age and the number of prior pregnancies do not appear to be factors that influence the occurrence of this type of twins. However, use of assisted reproductive techniques (for example, in vitro fertilization) may result in an increased risk for conjoined twins.

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