Conjoined twins are a rare form of identical twins who are physically connected at birth. This condition arises when a single fertilized egg, destined to develop into identical twins, does not completely separate during early embryonic development. The degree and location of their physical fusion can vary widely, ranging from sharing skin and muscle to sharing vital organs. As a result of their origin from a single egg, conjoined twins always share the same sex and possess identical genetic material.
Understanding the Incidence
The occurrence of conjoined twins is an exceptionally rare phenomenon, making it one of the least common types of twin pregnancies. Globally, estimates suggest that conjoined twins occur in approximately 1 in every 50,000 to 200,000 births. This incidence rate reflects the number of pregnancies where conjoined twins are present, encompassing both live births and stillbirths. A significant proportion, around half, of conjoined twin pregnancies result in stillbirth, and an additional one-third of those born alive may not survive beyond 24 hours.
How Conjoined Twins Form
Conjoined twins originate from a single fertilized egg that begins to divide, aiming to form identical twins, but the division process remains incomplete. This occurs when the monozygotic twin pregnancy cleaves more than 13 days after fertilization. Normally, identical twins form when a single embryo splits entirely into two separate entities. In the case of conjoined twins, this separation halts prematurely, leaving the developing fetuses physically connected. The precise timing of this incomplete division during the early embryonic stage dictates the extent and location of the fusion.
The dominant theory suggests that this incomplete fission leads to the shared body structures. If the separation occurs too late, after the embryonic disk has already begun to differentiate and form organ systems, the twins will remain joined. The failure of complete separation is not a result of two separate embryos fusing, but rather an interruption in the normal identical twinning process.
Variations in Conjoined Twin Types
Conjoined twins are classified based on the specific anatomical site where their bodies are fused. The most common types include fusion at the chest and abdomen, often sharing organs within these regions. For example, thoracopagus twins are joined at the chest, frequently sharing a heart. Omphalopagus twins are connected at the lower abdomen, commonly sharing a liver or parts of their digestive system.
Other forms of conjoining include pygopagus twins, who are joined at the buttocks, and craniopagus twins, who are fused at the skull. Each type presents unique medical challenges due to the varying degrees of shared organs and bodily systems. The specific point of fusion determines the complexity of their shared anatomy and influences potential medical interventions.
What Influences Their Occurrence
Conjoined twins are primarily considered a spontaneous developmental accident, not linked to specific external or hereditary factors. No clear evidence suggests that genetics or environmental exposures significantly increase the likelihood of conjoined twinning. This phenomenon is largely unpredictable, often occurring without any known predisposing conditions in the parents.
While some studies have explored potential associations, the consensus remains that conjoined twinning is not typically influenced by maternal age, prior pregnancies, or lifestyle choices. Its spontaneous nature means that it is not generally preventable through genetic screening or changes in maternal behavior. It is a random event during early embryonic development.