The Science of Stephen King

Parasitic twins occur when a twin embryo begins developing in utero, but the pair does not fully separate, and one embryo maintains dominant development at the expense of its twin. Unlike conjoined twins, one ceases development during gestation and is vestigial to a mostly fully formed, otherwise healthy individual twin. The undeveloped twin is defined as parasitic, rather than conjoined, because it is incompletely formed or wholly dependent on the body functions of the complete fetus. The independent twin is called the autosite.

In The Dark Half, Thad is the autosite, and the remains of the twin in his brain, who later takes on the identity of his pseudonym George Stark, is the parasite. The 1989 novel, and its 1993 film version starring Timothy Hutton, are hardly the first or only appearances of such a phenomenon in media. The low-budget horror film Basket Case (1982) centers on Duane (Kevin VanHentenryk), who carries around his removed parasitic twin in a basket as he seeks his fortune in New York City. As you might have guessed, Duane’s deformed twin also has a hankering for mischief and murder! This same theme is also found in “Humbug,” a 1995 episode of The X-Files (1993–2018), when once again a parasitic twin, thought to have no autonomy, detaches from his autosite every night to feast on the unsuspecting denizens of a trailer park.

The reality is that babies born with parasitic twins in the modern era no longer have to endure a demeaning life in the freak show circuit. Because of knowledgeable surgeons and advances in both technology and societal understanding, children can have their extra appendages removed or altered so that they can live full, happy lives. But, while this is wholly possible, the surgeries needed to detach the autosite from their parasitic twin can be life-threatening. In the case of ten-month-old Dominique in 2017, her parasitic twin was not just an anomaly of physical appearance, it was risking her life by demanding too much blood from her small heart. Born with her twin’s waist, legs, and feet growing from her back, there was a strain on Dominique’s spine as well. From Côte d’Ivoire, Dominique was fortunate to receive help from a host family in the United States, where she was given a grueling surgery at the Advocate Children’s Hospital in Illinois. This complex procedure included consultation from over fifty physicians before it even began. As described to Ashley Strickland for CNN, this was not a rote task for the highly accomplished staff.

“It allowed us to come up with a plan of attack how we could safely and effectively remove this very complex part attached to this little baby’s spine and end up with a healthy and happy child at the end of the day,” said Dr. Frank Vicari, a pediatric plastic and reconstructive surgeon at Advocate. The team staged a mock operation to figure out who would be doing what at specific parts of the procedure. On March 8, the team worked for six hours to remove the entirety of the parasitic twin. They had to be careful to disconnect any nerves and blood vessels so that Dominique wouldn’t sustain damage, numbness, or paralysis. Through extensive planning, they were even able to remove it all in one piece. She is now two pounds lighter.4

After a successful surgery, Dominique was able to return home to her family in Cote d’Ivoire. Her doctors believe she will no longer be hindered by her unusual condition.

Another true case of parasitic twins recently occurred in India. Similar to the fictional case of Thad Beaumont, an unnamed teenage girl was born and grew through childhood without any clue as to the interloper in her body. Like Thad, the teenager, who complained of a heavy abdomen and feeling “full,” was actually suffering from “fetus in fetu.” This is when the parasitic tissue is found inside the body and does not present externally. Journalist Diane Galistan explained the rare surgery for the International Business Times. “The lump on her abdomen had been steadily expanding over the last five years. When surgically removed, the mass was approximately two-thirds the size of a full-term baby. And was composed of hairy, cheesy material, multiple teeth, and structures resembling limb buds.”5

Scientists have varied opinions on whether fetus in fetu is the absorption of a twin in the embryonic stage or simply a teratoma, an advanced tumor consisting of hair, skin, and bone. Obviously, Stephen King fancied the former explanation in his weaving of the horrific story of Thad and George.

Fetus in fetu is estimated to occur in one in every five hundred thousand live births.