How Do Conjoined Twins Form?

Examine the Causes, Risk Factors and Symptoms of Conjoined Twins

Herrin twins, before separation
Herrin twins, before separation in 2006. Photo reprinted with permission of Herrin family.

Conjoined twins are an extremely rare type of twins, specifically a form of monozygotic twins.  Monozygotic-- also known as identical -- twins form when a single fertilized egg splits into two embryos. This split can happen anytime after fertilization; as more time passes, the twins are more likely to share commonalities. Conjoined twins develop and are born with a physical connection. Like all monozygotic twins, conjoined twins are always same sex (either both boys or both girls).

It is estimated that seventy percent of conjoined twins are female.

Theories About How Conjoined Twins Form

Conjoined twins remain a rare and mysterious phenomenon. Just as it is not known what ​causes monozygotic twinning, it is not known exactly why conjoined twins form, and there is disagreement as to how they form. Two theories have been presented.

Fission Theory of Conjoined Twinning

First, If the split occurs more than twelve days post conception, the embryos may not fully divide and the embryo may form an anatomical connection, producing conjoined twins. This is described as the "fission theory." It is not known why the zygote divides, or why the process is interrupted so that the embryo remains connected and develops as conjoined twins.

Fusion Theory of Conjoined Twinning

Another theory regarding conjoined twinning is known as the "fusion" or "collision" theory. It suggests that stem cells from one twin adhere to like stem cells from the other twin, fusing together and developing conjoinedly.

 Again, there is no identified explanation for why this would happen.

The amount of connectivity between conjoined twins varies and determines the expected outcome for the individuals. Some conjoined twins are merely connected with a narrow band of skin and tissue, while others share complicated connections of vital organs and systems that impair their function.

Nearly half of conjoined twins are born stillborn and about a third survive for less than a day after birth. However, surgical separations of conjoined twins have produced many successful cases where both individuals are able to enjoy long, healthy lives. Some examples are Carl and Clarence Aguirre in 2003, Jade and Erin Buckles in 2004, Kendra and Maliyah Herrin in 2006, and A’zhari and A’zhiah Jones in 2013. 

What are the Risk Factors For Conjoined Twins?

It is difficult to identify risk factors for the occurrence of conjoined twins in a pregnancy, as there is uncertainty about how and why they occur, to begin with. Monozygotic twinning is not associated with genetic or hereditary factors, nor influenced by some of the causes of dizygotic twinning, such as advanced maternal age or fertility treatments. Although it's been suggested that environmental factors may have an impact, especially in less developed countries, there is a lack of conclusive evidence identifying specific factors. 

What are the Symptoms of Conjoined Twins?

There are no specific symptoms for conjoined twins, although some of the general signs and symptoms of a multiple pregnancy may arouse suspicion. Conjoined twins are identified in pregnancy via ultrasound.

Further assessment and diagnostics can be obtained with an imaging technology such as MRI.


Spencher, R. "Theoretical and analytical embryology of conjoined twins: part I: embryogenesis." Clinical Anatomy, Vol. 13, Issue 1, 2000, pg. 36-53.

Quigley, C. Conjoined Twins: A Historical, Biological and Ethical Issues Encyclopedia, McFarland & Company (2003).  

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