Graveyard Shift Everything You Ever Wanted To Know About Parasitic Twins  

Crystal Brackett
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In the wide world of medical anomalies, conjoined twins present a unique condition. Among those extraordinary cases, some conjoined twins experience an even rarer condition: a parasitic twin. Unlike conjoined twins, in which the twins are attached yet possess functional organs, a parasitic twin consists of a dependent set of appendages that requires a healthy host - in this case, the living twin - to survive. Because the host twin ends up diverting resources from their body to the parasitic twin's, they find themselves at risk for all sorts of medical complications. 

As with most physiological phenomena, tons of questions surround the medical condition. What causes a parasitic twin? And better yet, how does modern medicine assist individuals with this condition? When you dive deeper into medical facts about parasitic twins, you see the various ways the condition expresses itself. 

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Photo:  Nate Davis/Flickr/CC BY-SA 2.0

Roughly One In 1 Million Births Are Parasitic Twins

Also called heteropagus twins, parasitic twins are very rare anomalies. The likely occurrence of parasitic twins lies around one per 1 million live births - notably smaller than the rate of conjoined twins, which is about one in 50,000 to one in 200,000.

These numbers are an estimate, given the difficulty of establishing an accurate global count.

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Photo: Johann Schenk/Wikimedia Commons/Public Domain

Asymmetrical Twins Consist Of A Developed And Underdeveloped Twin

Asymmetrical twins consist of a partly or fully developed twin - the autosite - and an underdeveloped, parasitic twin. The parasitic twin relies on the cardiovascular system of the developed twin to survive. Though sometimes comprised of an upper or lower body part, such as a head or torso, a parasitic twin typically takes the form of extra arms or legs. 

In this illustration, the man with the torso and limbs protruding from his abdomen is an autosite with a vestigial twin. This differs from the right two sets of conjoined twins, where both twins are technically developed and independent of their other.

Parasitic Twins Are The Result Of An Egg That Fails To Split

Parasitic twins differ from conjoined twins and vanishing twins, but the condition expresses itself in similar ways to both. Although the term "conjoined twins" applies to parasitic twins, as well, conjoined twins are typically fully developed and separately functional, unlike the independent autosite and its dependent parasite. On the other hand, vanishing twins are the result of a miscarriage of one twin, after which the other twin absorbs the fetal tissue. 

Scientists theorize parasitic twins may arise when an egg fails to split correctly during fertilization. According to other experts, an excess of the Sonic hedgehog gene (SHH), a vital protein in development, can cause uncontrolled growth in an embryo.

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Photo: Reginald J. Gladstone/Wikimedia Commons/Public Domain

A Parasitic Twin Can Kill The Autosite During Pregnancy

The autosite is responsible for pumping blood and providing energy for the parasitic twin, a relationship that can prove deadly in the womb. A parasitic twin without a heart, like the one shown here, causes Twin Reversed Arterial Profusion sequence, or TRAP, in which the healthy twin pumps blood in a "reversed" direction to the parasite. TRAP may lead to premature birth and even the autosite's heart failure, but modern medicine offers a few effective measures to prevent it.