Am I carrying a twin of my own?

Have you ever felt like something was missing in your life? Not the obvious kind of missing, like the absence of a spouse or a fancy car. Rather, it’s something that’s always been off. Maybe you invented a secret language no one could understand, but never had someone to share it with. If that’s the case, you might be experiencing the phenomenon of a disappearing twin.

While it may sound like a Stephen King novel, vanishing twins are actually a frequent occurrence. Experts estimate that 1 in 8 pregnancies involve twins, yet only 1 in 80 lead to the birth of twins [source: Goldman]. So, where do all those unborn twins go? They pass away early in the pregnancy and are absorbed back into the mother’s body. Bleeding during the first trimester can sometimes indicate this process is happening.

A 2007 study revealed that the surviving twin—who becomes the sole participant in the birth process—shows no developmental difference from the other twin who vanishes. However, many online communities exist for individuals who feel a sense of loss for their missing twin [source: Anand et al.]. In 2005, American cyclist Tyler Hamilton was accused of blood doping, where athletes enhance their performance by using someone else's blood; Hamilton defended himself by claiming the blood cells must have come from his missing twin [source: Kolata]. Regardless, Hamilton was suspended from the sport.

Hamilton's case could be seen as an exaggerated form of sibling rivalry; after all, while Jan Brady felt overshadowed by her sister Marcia, she never blamed her issues on a vanishing twin. However, Jan Brady—and even Peter Brady—might be shocked to learn that some twins don’t entirely disappear. Instead, they get absorbed into their twin's body. With how common vanishing twins are, one might wonder: Am I carrying my own twin?

Fetus in Fetu

Fetus in fetu is an uncommon condition where a fetus becomes enclosed within its twin while still in the womb. Unlike fraternal twins, which come from two separate fertilized eggs, identical twins are the result of one fertilized egg splitting. This splitting process doesn't always go perfectly, leading to instances like conjoined twins. Fetus in fetu occurs early in pregnancy when one fetus fails to separate properly, and instead becomes trapped inside the other. The enclosed fetus then behaves like a parasite, relying on the twin's blood supply for survival. If it grows, it can cause significant pain to the host twin.

Fetus in fetu is typically discovered when the host twin is still an infant, but there have been cases where people in their 30s and 40s have carried their own twin. One of the most notable cases occurred in India, where a 36-year-old man appeared pregnant. When doctors removed what they thought was a tumor, they found a malformed fetus inside, complete with limbs, genitalia, and hair [source: ABC News]. However, such cases are rare; about 90 percent of instances are found before the infant reaches 18 months of age [source: Hoeffel et al.]. These infants usually present with an abdominal mass, though the lump can sometimes appear in other areas like the brain or scrotum.

For those of you contemplating your reflection in the mirror, it’s important to remember that fetus in fetu is incredibly rare, with fewer than 100 documented cases in medical literature [source: Newsome]. Doctors estimate it occurs in approximately 1 out of every 500,000 births [source: Goldman]. It affects both males and females equally, and while it generally involves just one fetus, sometimes multiple fetuses are absorbed. When removed, these fetuses typically measure between 1.5 and 9.5 inches (4 to 24.5 centimeters) and weigh between 0.04 ounces (1.2 grams) and 4 pounds (1.8 kilograms) [source: Hoeffel et al.]. Despite various body parts being present, the fetus is always anencephalic, meaning it lacks major parts of its brain and skull. Once the mass is surgically removed, the host twin typically recovers fully.

It’s important to note that some experts believe that what seems to be fetus in fetu might actually be a teratoma. A teratoma is a type of tumor that can sometimes resemble a malformed fetus, but it is actually a mass of cells that may develop skin, hair, and even teeth. In some cases, it's difficult to differentiate between the two, but teratomas are more common than fetus in fetu. If the mass shows signs of a vertebral column, however, the case is considered fetus in fetu.