World Conjoined Twins Day Date in the current year: November 24, 2024

Conjoined twins, also known as Siamese twins, are identical twins joined together in the womb. The term Siamese twins came to be associated with conjoined twins because of Chang and Eng Bunker, a pair of conjoined twins born in Siam (now Thailand) in 1811. They traveled extensively around the world, participating in “freak shows” and drawing public attention to their rare condition.

The generally accepted explanation for the cause of the conditions is fission, meaning that conjoined twins develop from a single fertilized egg that does not fully separate into two identical twins during the early stages of embryonic development. There are different types of conjoined twins depending on where their bodies are joined (abdomen, chest, head, pelvis, spine, etc.). The most common are:

• Thoracopagus: fused from the upper to the lower chest, usually sharing the heart.
• Thoraco-omphalopagus: fused from the upper chest to the lower abdomen, always sharing the heart.
• Omphalopagus: fused at the lower abdomen, not sharing the heart but often sharing the liver and other organs.
• Parasitic twins: asymmetrically fused twins, where one is smaller, less developed, and dependent on the other twin’s organs for survival.
• Craniopagus: fused at the base, front, or side of the head, but separate bodies.

Conjoined twins are extremely rare; their incidence ranges from one in 50,000 to one in 200,000 births. There appears to be no association with heredity, race, maternal age, or number of previous births, but conjoined twins are more often female than male. About 60% of conjoined twins are stillborn or die within 24 hours of birth.

The survival rate of conjoined twins usually depends on where the twins’ bodies are joined and the number of internal organs they share. For example, the survival rate for craniopagus twins or thoracopagus twins is poor, but the survival rate for omphalopagus twins is over 80%.

In some cases, surgery can be performed to separate conjoined twins, although it should be noted that this is extremely risky and often results in the death of at least one of the twins, especially if they share a heart or other vital organ. Because of this, there has been debate about whether it is ethical to separate conjoined twins who can survive if not separated.

Research has shown that conjoined twins can have a high quality of life even if they are not separated. Notable examples include Chang and Eng Bunker, Abby and Brittany Hensel, and Lori and George Schappell (George was born Dori Schappell and came out as a trans man in his 40s). Just like regular twins, conjoined twins can have very different personalities and live individual lives to the extent possible. However, they face unique challenges that have a significant impact on their well-being.

The United Nations General Assembly established World Conjoined Twins Day in 2024 to raise awareness of conjoined twins and to promote their well-being, social inclusion, and human rights as distinct individuals who happen to share a body.