Is There A Way To Prevent Conjoined Twinning?

Conjoined Twins
Conjoined Twins. Getty Images North America

Conjoined Twins FAQ: Is There A Way To Prevent Conjoined Twinning?

Question.: "Is there any reason why twins are conjoined? Is there any way to prevent it before birth?"

Answer: The short answer to this question is "No." There is no known way to prevent conjoined twinning. But in order to fully address the issue of preventing conjoined twinning, it's important to understand a bit more about conjoined twins.

This is a difficult question to answer without first looking at how and why conjoined twins happen.

Conjoined twins are a type of monozygotic twins. That is, they originate as a single zygote that splits into two. (A zygote is a fertilized egg.) At some point after conception, as the zygote travels towards the uterus for implantation, the cells divide and combine. And in the case of monozygotic twins, the cells split and form as two blastocysts, resulting in twins. This split can occur within a few days, but in some cases it is delayed and doesn't split for twelve days or more. That's when conjoined twins form, twins that develop in the womb with a physical connection. As the name suggests, they are literally two individuals joined together. They may share skin and tissue, or they may develop with shared organs and limbs. Conjoined twins are very rare, occurring in only 1 out of 200,000 live births.

It is not known what causes monozygotic twinning, and likewise, no one is absolutely certain what causes conjoined twinning. Although there are theories to explain how conjoined twins form, there is a great deal of uncertainty about the exact cause. There is no clearly identified reason to explain why some fertilized eggs split, developing into two individuals.

Nor is there any explanation as to why the split would be delayed or incomplete, resulting in conjoined twins. Conjoined twinning can't be attributed to genetics, the mother's behavior, trauma, a virus, illness, environmental issues or any other factors known at this time. 

Since it's not known what causes the situation that produces conjoined twins, there's also no known way to prevent it from happening. Monozygotic twinning -- and conjoined twinning -- remain somewhat of an unexplained mystery. 

Conjoined twins, also known as Siamese twins, are a rare type of monozygotic (identical) twins. Find more answers to Frequently Asked Questions About Conjoined Twins.


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: An Historical, Biological and Ethical Issues Encyclopedia, McFarland & Company (2003).  

Facts about conjoined twins. University of Maryland Medical Center. Accessed September 3, 2015. 

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